aboutsummaryrefslogtreecommitdiffstats
path: root/activerecord/lib/active_record/associations.rb
blob: 1b9b725dd40acd40acad14515b08f059a393ce56 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
require 'active_support/core_ext/array/wrap'
require 'active_support/core_ext/enumerable'
require 'active_support/core_ext/module/delegation'
require 'active_support/core_ext/object/blank'
require 'active_support/core_ext/string/conversions'
require 'active_support/core_ext/module/remove_method'

module ActiveRecord
  class InverseOfAssociationNotFoundError < ActiveRecordError #:nodoc:
    def initialize(reflection, associated_class = nil)
      super("Could not find the inverse association for #{reflection.name} (#{reflection.options[:inverse_of].inspect} in #{associated_class.nil? ? reflection.class_name : associated_class.name})")
    end
  end

  class HasManyThroughAssociationNotFoundError < ActiveRecordError #:nodoc:
    def initialize(owner_class_name, reflection)
      super("Could not find the association #{reflection.options[:through].inspect} in model #{owner_class_name}")
    end
  end

  class HasManyThroughAssociationPolymorphicError < ActiveRecordError #:nodoc:
    def initialize(owner_class_name, reflection, source_reflection)
      super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' on the polymorphic object '#{source_reflection.class_name}##{source_reflection.name}'.")
    end
  end

  class HasManyThroughAssociationPointlessSourceTypeError < ActiveRecordError #:nodoc:
    def initialize(owner_class_name, reflection, source_reflection)
      super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' with a :source_type option if the '#{reflection.through_reflection.class_name}##{source_reflection.name}' is not polymorphic.  Try removing :source_type on your association.")
    end
  end

  class HasManyThroughSourceAssociationNotFoundError < ActiveRecordError #:nodoc:
    def initialize(reflection)
      through_reflection      = reflection.through_reflection
      source_reflection_names = reflection.source_reflection_names
      source_associations     = reflection.through_reflection.klass.reflect_on_all_associations.collect { |a| a.name.inspect }
      super("Could not find the source association(s) #{source_reflection_names.collect(&:inspect).to_sentence(:two_words_connector => ' or ', :last_word_connector => ', or ', :locale => :en)} in model #{through_reflection.klass}.  Try 'has_many #{reflection.name.inspect}, :through => #{through_reflection.name.inspect}, :source => <name>'.  Is it one of #{source_associations.to_sentence(:two_words_connector => ' or ', :last_word_connector => ', or ', :locale => :en)}?")
    end
  end

  class HasManyThroughSourceAssociationMacroError < ActiveRecordError #:nodoc:
    def initialize(reflection)
      through_reflection = reflection.through_reflection
      source_reflection  = reflection.source_reflection
      super("Invalid source reflection macro :#{source_reflection.macro}#{" :through" if source_reflection.options[:through]} for has_many #{reflection.name.inspect}, :through => #{through_reflection.name.inspect}.  Use :source to specify the source reflection.")
    end
  end

  class HasManyThroughCantAssociateThroughHasOneOrManyReflection < ActiveRecordError #:nodoc:
    def initialize(owner, reflection)
      super("Cannot modify association '#{owner.class.name}##{reflection.name}' because the source reflection class '#{reflection.source_reflection.class_name}' is associated to '#{reflection.through_reflection.class_name}' via :#{reflection.source_reflection.macro}.")
    end
  end

  class HasManyThroughCantAssociateNewRecords < ActiveRecordError #:nodoc:
    def initialize(owner, reflection)
      super("Cannot associate new records through '#{owner.class.name}##{reflection.name}' on '#{reflection.source_reflection.class_name rescue nil}##{reflection.source_reflection.name rescue nil}'. Both records must have an id in order to create the has_many :through record associating them.")
    end
  end

  class HasManyThroughCantDissociateNewRecords < ActiveRecordError #:nodoc:
    def initialize(owner, reflection)
      super("Cannot dissociate new records through '#{owner.class.name}##{reflection.name}' on '#{reflection.source_reflection.class_name rescue nil}##{reflection.source_reflection.name rescue nil}'. Both records must have an id in order to delete the has_many :through record associating them.")
    end
  end

  class HasAndBelongsToManyAssociationWithPrimaryKeyError < ActiveRecordError #:nodoc:
    def initialize(reflection)
      super("Primary key is not allowed in a has_and_belongs_to_many join table (#{reflection.options[:join_table]}).")
    end
  end

  class HasAndBelongsToManyAssociationForeignKeyNeeded < ActiveRecordError #:nodoc:
    def initialize(reflection)
      super("Cannot create self referential has_and_belongs_to_many association on '#{reflection.class_name rescue nil}##{reflection.name rescue nil}'. :association_foreign_key cannot be the same as the :foreign_key.")
    end
  end

  class EagerLoadPolymorphicError < ActiveRecordError #:nodoc:
    def initialize(reflection)
      super("Can not eagerly load the polymorphic association #{reflection.name.inspect}")
    end
  end

  class ReadOnlyAssociation < ActiveRecordError #:nodoc:
    def initialize(reflection)
      super("Can not add to a has_many :through association.  Try adding to #{reflection.through_reflection.name.inspect}.")
    end
  end

  # This error is raised when trying to destroy a parent instance in N:1 or 1:1 associations
  # (has_many, has_one) when there is at least 1 child associated instance.
  # ex: if @project.tasks.size > 0, DeleteRestrictionError will be raised when trying to destroy @project
  class DeleteRestrictionError < ActiveRecordError #:nodoc:
    def initialize(reflection)
      super("Cannot delete record because of dependent #{reflection.name}")
    end
  end

  # See ActiveRecord::Associations::ClassMethods for documentation.
  module Associations # :nodoc:
    extend ActiveSupport::Concern

    # These classes will be loaded when associations are created.
    # So there is no need to eager load them.
    autoload :AssociationCollection, 'active_record/associations/association_collection'
    autoload :AssociationProxy, 'active_record/associations/association_proxy'
    autoload :BelongsToAssociation, 'active_record/associations/belongs_to_association'
    autoload :BelongsToPolymorphicAssociation, 'active_record/associations/belongs_to_polymorphic_association'
    autoload :HasAndBelongsToManyAssociation, 'active_record/associations/has_and_belongs_to_many_association'
    autoload :HasManyAssociation, 'active_record/associations/has_many_association'
    autoload :HasManyThroughAssociation, 'active_record/associations/has_many_through_association'
    autoload :HasOneAssociation, 'active_record/associations/has_one_association'
    autoload :HasOneThroughAssociation, 'active_record/associations/has_one_through_association'

    # Clears out the association cache
    def clear_association_cache #:nodoc:
      self.class.reflect_on_all_associations.to_a.each do |assoc|
        instance_variable_set "@#{assoc.name}", nil
      end unless self.new_record?
    end

    private
      # Gets the specified association instance if it responds to :loaded?, nil otherwise.
      def association_instance_get(name)
        ivar = "@#{name}"
        if instance_variable_defined?(ivar)
          association = instance_variable_get(ivar)
          association if association.respond_to?(:loaded?)
        end
      end

      # Set the specified association instance.
      def association_instance_set(name, association)
        instance_variable_set("@#{name}", association)
      end

    # Associations are a set of macro-like class methods for tying objects together through foreign keys. They express relationships like
    # "Project has one Project Manager" or "Project belongs to a Portfolio". Each macro adds a number of methods to the class which are
    # specialized according to the collection or association symbol and the options hash. It works much the same way as Ruby's own <tt>attr*</tt>
    # methods. Example:
    #
    #   class Project < ActiveRecord::Base
    #     belongs_to              :portfolio
    #     has_one                 :project_manager
    #     has_many                :milestones
    #     has_and_belongs_to_many :categories
    #   end
    #
    # The project class now has the following methods (and more) to ease the traversal and manipulation of its relationships:
    # * <tt>Project#portfolio, Project#portfolio=(portfolio), Project#portfolio.nil?</tt>
    # * <tt>Project#project_manager, Project#project_manager=(project_manager), Project#project_manager.nil?,</tt>
    # * <tt>Project#milestones.empty?, Project#milestones.size, Project#milestones, Project#milestones<<(milestone),</tt>
    #   <tt>Project#milestones.delete(milestone), Project#milestones.find(milestone_id), Project#milestones.find(:all, options),</tt>
    #   <tt>Project#milestones.build, Project#milestones.create</tt>
    # * <tt>Project#categories.empty?, Project#categories.size, Project#categories, Project#categories<<(category1),</tt>
    #   <tt>Project#categories.delete(category1)</tt>
    #
    # === A word of warning
    #
    # Don't create associations that have the same name as instance methods of ActiveRecord::Base. Since the association
    # adds a method with that name to its model, it will override the inherited method and break things.
    # For instance, +attributes+ and +connection+ would be bad choices for association names.
    #
    # == Auto-generated methods
    #
    # === Singular associations (one-to-one)
    #                                     |            |  belongs_to  |
    #   generated methods                 | belongs_to | :polymorphic | has_one
    #   ----------------------------------+------------+--------------+---------
    #   other                             |     X      |      X       |    X
    #   other=(other)                     |     X      |      X       |    X
    #   build_other(attributes={})        |     X      |              |    X
    #   create_other(attributes={})       |     X      |              |    X
    #   other.create!(attributes={})      |            |              |    X
    #
    # ===Collection associations (one-to-many / many-to-many)
    #                                     |       |          | has_many
    #   generated methods                 | habtm | has_many | :through
    #   ----------------------------------+-------+----------+----------
    #   others                            |   X   |    X     |    X
    #   others=(other,other,...)          |   X   |    X     |    X
    #   other_ids                         |   X   |    X     |    X
    #   other_ids=(id,id,...)             |   X   |    X     |    X
    #   others<<                          |   X   |    X     |    X
    #   others.push                       |   X   |    X     |    X
    #   others.concat                     |   X   |    X     |    X
    #   others.build(attributes={})       |   X   |    X     |    X
    #   others.create(attributes={})      |   X   |    X     |    X
    #   others.create!(attributes={})     |   X   |    X     |    X
    #   others.size                       |   X   |    X     |    X
    #   others.length                     |   X   |    X     |    X
    #   others.count                      |   X   |    X     |    X
    #   others.sum(args*,&block)          |   X   |    X     |    X
    #   others.empty?                     |   X   |    X     |    X
    #   others.clear                      |   X   |    X     |    X
    #   others.delete(other,other,...)    |   X   |    X     |    X
    #   others.delete_all                 |   X   |    X     |
    #   others.destroy_all                |   X   |    X     |    X
    #   others.find(*args)                |   X   |    X     |    X
    #   others.find_first                 |   X   |          |
    #   others.exists?                    |   X   |    X     |    X
    #   others.uniq                       |   X   |    X     |    X
    #   others.reset                      |   X   |    X     |    X
    #
    # == Cardinality and associations
    #
    # Active Record associations can be used to describe one-to-one, one-to-many and many-to-many
    # relationships between models. Each model uses an association to describe its role in
    # the relation. The +belongs_to+ association is always used in the model that has
    # the foreign key.
    #
    # === One-to-one
    #
    # Use +has_one+ in the base, and +belongs_to+ in the associated model.
    #
    #   class Employee < ActiveRecord::Base
    #     has_one :office
    #   end
    #   class Office < ActiveRecord::Base
    #     belongs_to :employee    # foreign key - employee_id
    #   end
    #
    # === One-to-many
    #
    # Use +has_many+ in the base, and +belongs_to+ in the associated model.
    #
    #   class Manager < ActiveRecord::Base
    #     has_many :employees
    #   end
    #   class Employee < ActiveRecord::Base
    #     belongs_to :manager     # foreign key - manager_id
    #   end
    #
    # === Many-to-many
    #
    # There are two ways to build a many-to-many relationship.
    #
    # The first way uses a +has_many+ association with the <tt>:through</tt> option and a join model, so
    # there are two stages of associations.
    #
    #   class Assignment < ActiveRecord::Base
    #     belongs_to :programmer  # foreign key - programmer_id
    #     belongs_to :project     # foreign key - project_id
    #   end
    #   class Programmer < ActiveRecord::Base
    #     has_many :assignments
    #     has_many :projects, :through => :assignments
    #   end
    #   class Project < ActiveRecord::Base
    #     has_many :assignments
    #     has_many :programmers, :through => :assignments
    #   end
    #
    # For the second way, use +has_and_belongs_to_many+ in both models. This requires a join table
    # that has no corresponding model or primary key.
    #
    #   class Programmer < ActiveRecord::Base
    #     has_and_belongs_to_many :projects       # foreign keys in the join table
    #   end
    #   class Project < ActiveRecord::Base
    #     has_and_belongs_to_many :programmers    # foreign keys in the join table
    #   end
    #
    # Choosing which way to build a many-to-many relationship is not always simple.
    # If you need to work with the relationship model as its own entity,
    # use <tt>has_many :through</tt>. Use +has_and_belongs_to_many+ when working with legacy schemas or when
    # you never work directly with the relationship itself.
    #
    # == Is it a +belongs_to+ or +has_one+ association?
    #
    # Both express a 1-1 relationship. The difference is mostly where to place the foreign key, which goes on the table for the class
    # declaring the +belongs_to+ relationship. Example:
    #
    #   class User < ActiveRecord::Base
    #     # I reference an account.
    #     belongs_to :account
    #   end
    #
    #   class Account < ActiveRecord::Base
    #     # One user references me.
    #     has_one :user
    #   end
    #
    # The tables for these classes could look something like:
    #
    #   CREATE TABLE users (
    #     id int(11) NOT NULL auto_increment,
    #     account_id int(11) default NULL,
    #     name varchar default NULL,
    #     PRIMARY KEY  (id)
    #   )
    #
    #   CREATE TABLE accounts (
    #     id int(11) NOT NULL auto_increment,
    #     name varchar default NULL,
    #     PRIMARY KEY  (id)
    #   )
    #
    # == Unsaved objects and associations
    #
    # You can manipulate objects and associations before they are saved to the database, but there is some special behavior you should be
    # aware of, mostly involving the saving of associated objects.
    #
    # Unless you set the :autosave option on a <tt>has_one</tt>, <tt>belongs_to</tt>,
    # <tt>has_many</tt>, or <tt>has_and_belongs_to_many</tt> association. Setting it
    # to +true+ will _always_ save the members, whereas setting it to +false+ will
    # _never_ save the members.
    #
    # === One-to-one associations
    #
    # * Assigning an object to a +has_one+ association automatically saves that object and the object being replaced (if there is one), in
    #   order to update their primary keys - except if the parent object is unsaved (<tt>new_record? == true</tt>).
    # * If either of these saves fail (due to one of the objects being invalid) the assignment statement returns +false+ and the assignment
    #   is cancelled.
    # * If you wish to assign an object to a +has_one+ association without saving it, use the <tt>association.build</tt> method (documented below).
    # * Assigning an object to a +belongs_to+ association does not save the object, since the foreign key field belongs on the parent. It
    #   does not save the parent either.
    #
    # === Collections
    #
    # * Adding an object to a collection (+has_many+ or +has_and_belongs_to_many+) automatically saves that object, except if the parent object
    #   (the owner of the collection) is not yet stored in the database.
    # * If saving any of the objects being added to a collection (via <tt>push</tt> or similar) fails, then <tt>push</tt> returns +false+.
    # * You can add an object to a collection without automatically saving it by using the <tt>collection.build</tt> method (documented below).
    # * All unsaved (<tt>new_record? == true</tt>) members of the collection are automatically saved when the parent is saved.
    #
    # === Association callbacks
    #
    # Similar to the normal callbacks that hook into the lifecycle of an Active Record object, you can also define callbacks that get
    # triggered when you add an object to or remove an object from an association collection. Example:
    #
    #   class Project
    #     has_and_belongs_to_many :developers, :after_add => :evaluate_velocity
    #
    #     def evaluate_velocity(developer)
    #       ...
    #     end
    #   end
    #
    # It's possible to stack callbacks by passing them as an array. Example:
    #
    #   class Project
    #     has_and_belongs_to_many :developers, :after_add => [:evaluate_velocity, Proc.new { |p, d| p.shipping_date = Time.now}]
    #   end
    #
    # Possible callbacks are: +before_add+, +after_add+, +before_remove+ and +after_remove+.
    #
    # Should any of the +before_add+ callbacks throw an exception, the object does not get added to the collection. Same with
    # the +before_remove+ callbacks; if an exception is thrown the object doesn't get removed.
    #
    # === Association extensions
    #
    # The proxy objects that control the access to associations can be extended through anonymous modules. This is especially
    # beneficial for adding new finders, creators, and other factory-type methods that are only used as part of this association.
    # Example:
    #
    #   class Account < ActiveRecord::Base
    #     has_many :people do
    #       def find_or_create_by_name(name)
    #         first_name, last_name = name.split(" ", 2)
    #         find_or_create_by_first_name_and_last_name(first_name, last_name)
    #       end
    #     end
    #   end
    #
    #   person = Account.find(:first).people.find_or_create_by_name("David Heinemeier Hansson")
    #   person.first_name # => "David"
    #   person.last_name  # => "Heinemeier Hansson"
    #
    # If you need to share the same extensions between many associations, you can use a named extension module. Example:
    #
    #   module FindOrCreateByNameExtension
    #     def find_or_create_by_name(name)
    #       first_name, last_name = name.split(" ", 2)
    #       find_or_create_by_first_name_and_last_name(first_name, last_name)
    #     end
    #   end
    #
    #   class Account < ActiveRecord::Base
    #     has_many :people, :extend => FindOrCreateByNameExtension
    #   end
    #
    #   class Company < ActiveRecord::Base
    #     has_many :people, :extend => FindOrCreateByNameExtension
    #   end
    #
    # If you need to use multiple named extension modules, you can specify an array of modules with the <tt>:extend</tt> option.
    # In the case of name conflicts between methods in the modules, methods in modules later in the array supercede
    # those earlier in the array. Example:
    #
    #   class Account < ActiveRecord::Base
    #     has_many :people, :extend => [FindOrCreateByNameExtension, FindRecentExtension]
    #   end
    #
    # Some extensions can only be made to work with knowledge of the association proxy's internals.
    # Extensions can access relevant state using accessors on the association proxy:
    #
    # * +proxy_owner+ - Returns the object the association is part of.
    # * +proxy_reflection+ - Returns the reflection object that describes the association.
    # * +proxy_target+ - Returns the associated object for +belongs_to+ and +has_one+, or the collection of associated objects for +has_many+ and +has_and_belongs_to_many+.
    #
    # === Association Join Models
    #
    # Has Many associations can be configured with the <tt>:through</tt> option to use an explicit join model to retrieve the data.  This
    # operates similarly to a +has_and_belongs_to_many+ association.  The advantage is that you're able to add validations,
    # callbacks, and extra attributes on the join model.  Consider the following schema:
    #
    #   class Author < ActiveRecord::Base
    #     has_many :authorships
    #     has_many :books, :through => :authorships
    #   end
    #
    #   class Authorship < ActiveRecord::Base
    #     belongs_to :author
    #     belongs_to :book
    #   end
    #
    #   @author = Author.find :first
    #   @author.authorships.collect { |a| a.book } # selects all books that the author's authorships belong to.
    #   @author.books                              # selects all books by using the Authorship join model
    #
    # You can also go through a +has_many+ association on the join model:
    #
    #   class Firm < ActiveRecord::Base
    #     has_many   :clients
    #     has_many   :invoices, :through => :clients
    #   end
    #
    #   class Client < ActiveRecord::Base
    #     belongs_to :firm
    #     has_many   :invoices
    #   end
    #
    #   class Invoice < ActiveRecord::Base
    #     belongs_to :client
    #   end
    #
    #   @firm = Firm.find :first
    #   @firm.clients.collect { |c| c.invoices }.flatten # select all invoices for all clients of the firm
    #   @firm.invoices                                   # selects all invoices by going through the Client join model.
    #
    # Similarly you can go through a +has_one+ association on the join model:
    #
    #   class Group < ActiveRecord::Base
    #     has_many   :users
    #     has_many   :avatars, :through => :users
    #   end
    #
    #   class User < ActiveRecord::Base
    #     belongs_to :group
    #     has_one    :avatar
    #   end
    #
    #   class Avatar < ActiveRecord::Base
    #     belongs_to :user
    #   end
    #
    #   @group = Group.first
    #   @group.users.collect { |u| u.avatar }.flatten # select all avatars for all users in the group
    #   @group.avatars                                # selects all avatars by going through the User join model.
    #
    # An important caveat with going through +has_one+ or +has_many+ associations on the join model is that these associations are
    # *read-only*.  For example, the following would not work following the previous example:
    #
    #   @group.avatars << Avatar.new                # this would work if User belonged_to Avatar rather than the other way around.
    #   @group.avatars.delete(@group.avatars.last)  # so would this
    #
    # === Polymorphic Associations
    #
    # Polymorphic associations on models are not restricted on what types of models they can be associated with.  Rather, they
    # specify an interface that a +has_many+ association must adhere to.
    #
    #   class Asset < ActiveRecord::Base
    #     belongs_to :attachable, :polymorphic => true
    #   end
    #
    #   class Post < ActiveRecord::Base
    #     has_many :assets, :as => :attachable         # The :as option specifies the polymorphic interface to use.
    #   end
    #
    #   @asset.attachable = @post
    #
    # This works by using a type column in addition to a foreign key to specify the associated record.  In the Asset example, you'd need
    # an +attachable_id+ integer column and an +attachable_type+ string column.
    #
    # Using polymorphic associations in combination with single table inheritance (STI) is a little tricky. In order
    # for the associations to work as expected, ensure that you store the base model for the STI models in the
    # type column of the polymorphic association. To continue with the asset example above, suppose there are guest posts
    # and member posts that use the posts table for STI. In this case, there must be a +type+ column in the posts table.
    #
    #   class Asset < ActiveRecord::Base
    #     belongs_to :attachable, :polymorphic => true
    #
    #     def attachable_type=(sType)
    #        super(sType.to_s.classify.constantize.base_class.to_s)
    #     end
    #   end
    #
    #   class Post < ActiveRecord::Base
    #     # because we store "Post" in attachable_type now :dependent => :destroy will work
    #     has_many :assets, :as => :attachable, :dependent => :destroy
    #   end
    #
    #   class GuestPost < Post
    #   end
    #
    #   class MemberPost < Post
    #   end
    #
    # == Caching
    #
    # All of the methods are built on a simple caching principle that will keep the result of the last query around unless specifically
    # instructed not to. The cache is even shared across methods to make it even cheaper to use the macro-added methods without
    # worrying too much about performance at the first go. Example:
    #
    #   project.milestones             # fetches milestones from the database
    #   project.milestones.size        # uses the milestone cache
    #   project.milestones.empty?      # uses the milestone cache
    #   project.milestones(true).size  # fetches milestones from the database
    #   project.milestones             # uses the milestone cache
    #
    # == Eager loading of associations
    #
    # Eager loading is a way to find objects of a certain class and a number of named associations. This is
    # one of the easiest ways of to prevent the dreaded 1+N problem in which fetching 100 posts that each need to display their author
    # triggers 101 database queries. Through the use of eager loading, the 101 queries can be reduced to 2. Example:
    #
    #   class Post < ActiveRecord::Base
    #     belongs_to :author
    #     has_many   :comments
    #   end
    #
    # Consider the following loop using the class above:
    #
    #   for post in Post.all
    #     puts "Post:            " + post.title
    #     puts "Written by:      " + post.author.name
    #     puts "Last comment on: " + post.comments.first.created_on
    #   end
    #
    # To iterate over these one hundred posts, we'll generate 201 database queries. Let's first just optimize it for retrieving the author:
    #
    #   for post in Post.find(:all, :include => :author)
    #
    # This references the name of the +belongs_to+ association that also used the <tt>:author</tt> symbol. After loading the posts, find
    # will collect the +author_id+ from each one and load all the referenced authors with one query. Doing so will cut down the number of queries from 201 to 102.
    #
    # We can improve upon the situation further by referencing both associations in the finder with:
    #
    #   for post in Post.find(:all, :include => [ :author, :comments ])
    #
    # This will load all comments with a single query. This reduces the total number of queries to 3. More generally the number of queries
    # will be 1 plus the number of associations named (except if some of the associations are polymorphic +belongs_to+ - see below).
    #
    # To include a deep hierarchy of associations, use a hash:
    #
    #   for post in Post.find(:all, :include => [ :author, { :comments => { :author => :gravatar } } ])
    #
    # That'll grab not only all the comments but all their authors and gravatar pictures.  You can mix and match
    # symbols, arrays and hashes in any combination to describe the associations you want to load.
    #
    # All of this power shouldn't fool you into thinking that you can pull out huge amounts of data with no performance penalty just because you've reduced
    # the number of queries. The database still needs to send all the data to Active Record and it still needs to be processed. So it's no
    # catch-all for performance problems, but it's a great way to cut down on the number of queries in a situation as the one described above.
    #
    # Since only one table is loaded at a time, conditions or orders cannot reference tables other than the main one. If this is the case
    # Active Record falls back to the previously used LEFT OUTER JOIN based strategy. For example
    #
    #   Post.find(:all, :include => [ :author, :comments ], :conditions => ['comments.approved = ?', true])
    #
    # This will result in a single SQL query with joins along the lines of: <tt>LEFT OUTER JOIN comments ON comments.post_id = posts.id</tt> and
    # <tt>LEFT OUTER JOIN authors ON authors.id = posts.author_id</tt>. Note that using conditions like this can have unintended consequences.
    # In the above example posts with no approved comments are not returned at all, because the conditions apply to the SQL statement as a whole
    # and not just to the association. You must disambiguate column references for this fallback to happen, for example
    # <tt>:order => "author.name DESC"</tt> will work but <tt>:order => "name DESC"</tt> will not.
    #
    # If you do want eager load only some members of an association it is usually more natural to <tt>:include</tt> an association
    # which has conditions defined on it:
    #
    #   class Post < ActiveRecord::Base
    #     has_many :approved_comments, :class_name => 'Comment', :conditions => ['approved = ?', true]
    #   end
    #
    #   Post.find(:all, :include => :approved_comments)
    #
    # This will load posts and eager load the +approved_comments+ association, which contains only those comments that have been approved.
    #
    # If you eager load an association with a specified <tt>:limit</tt> option, it will be ignored, returning all the associated objects:
    #
    #   class Picture < ActiveRecord::Base
    #     has_many :most_recent_comments, :class_name => 'Comment', :order => 'id DESC', :limit => 10
    #   end
    #
    #   Picture.find(:first, :include => :most_recent_comments).most_recent_comments # => returns all associated comments.
    #
    # When eager loaded, conditions are interpolated in the context of the model class, not the model instance.  Conditions are lazily interpolated
    # before the actual model exists.
    #
    # Eager loading is supported with polymorphic associations.
    #
    #   class Address < ActiveRecord::Base
    #     belongs_to :addressable, :polymorphic => true
    #   end
    #
    # A call that tries to eager load the addressable model
    #
    #   Address.find(:all, :include => :addressable)
    #
    # This will execute one query to load the addresses and load the addressables with one query per addressable type.
    # For example if all the addressables are either of class Person or Company then a total of 3 queries will be executed. The list of
    # addressable types to load is determined on the back of the addresses loaded. This is not supported if Active Record has to fallback
    # to the previous implementation of eager loading and will raise ActiveRecord::EagerLoadPolymorphicError. The reason is that the parent
    # model's type is a column value so its corresponding table name cannot be put in the +FROM+/+JOIN+ clauses of that query.
    #
    # == Table Aliasing
    #
    # Active Record uses table aliasing in the case that a table is referenced multiple times in a join.  If a table is referenced only once,
    # the standard table name is used.  The second time, the table is aliased as <tt>#{reflection_name}_#{parent_table_name}</tt>.  Indexes are appended
    # for any more successive uses of the table name.
    #
    #   Post.find :all, :joins => :comments
    #   # => SELECT ... FROM posts INNER JOIN comments ON ...
    #   Post.find :all, :joins => :special_comments # STI
    #   # => SELECT ... FROM posts INNER JOIN comments ON ... AND comments.type = 'SpecialComment'
    #   Post.find :all, :joins => [:comments, :special_comments] # special_comments is the reflection name, posts is the parent table name
    #   # => SELECT ... FROM posts INNER JOIN comments ON ... INNER JOIN comments special_comments_posts
    #
    # Acts as tree example:
    #
    #   TreeMixin.find :all, :joins => :children
    #   # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
    #   TreeMixin.find :all, :joins => {:children => :parent}
    #   # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
    #                               INNER JOIN parents_mixins ...
    #   TreeMixin.find :all, :joins => {:children => {:parent => :children}}
    #   # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
    #                               INNER JOIN parents_mixins ...
    #                               INNER JOIN mixins childrens_mixins_2
    #
    # Has and Belongs to Many join tables use the same idea, but add a <tt>_join</tt> suffix:
    #
    #   Post.find :all, :joins => :categories
    #   # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
    #   Post.find :all, :joins => {:categories => :posts}
    #   # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
    #                              INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories
    #   Post.find :all, :joins => {:categories => {:posts => :categories}}
    #   # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
    #                              INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories
    #                              INNER JOIN categories_posts categories_posts_join INNER JOIN categories categories_posts_2
    #
    # If you wish to specify your own custom joins using a <tt>:joins</tt> option, those table names will take precedence over the eager associations:
    #
    #   Post.find :all, :joins => :comments, :joins => "inner join comments ..."
    #   # => SELECT ... FROM posts INNER JOIN comments_posts ON ... INNER JOIN comments ...
    #   Post.find :all, :joins => [:comments, :special_comments], :joins => "inner join comments ..."
    #   # => SELECT ... FROM posts INNER JOIN comments comments_posts ON ...
    #                              INNER JOIN comments special_comments_posts ...
    #                              INNER JOIN comments ...
    #
    # Table aliases are automatically truncated according to the maximum length of table identifiers according to the specific database.
    #
    # == Modules
    #
    # By default, associations will look for objects within the current module scope. Consider:
    #
    #   module MyApplication
    #     module Business
    #       class Firm < ActiveRecord::Base
    #          has_many :clients
    #        end
    #
    #       class Client < ActiveRecord::Base; end
    #     end
    #   end
    #
    # When <tt>Firm#clients</tt> is called, it will in turn call <tt>MyApplication::Business::Client.find_all_by_firm_id(firm.id)</tt>.
    # If you want to associate with a class in another module scope, this can be done by specifying the complete class name.
    # Example:
    #
    #   module MyApplication
    #     module Business
    #       class Firm < ActiveRecord::Base; end
    #     end
    #
    #     module Billing
    #       class Account < ActiveRecord::Base
    #         belongs_to :firm, :class_name => "MyApplication::Business::Firm"
    #       end
    #     end
    #   end
    #
    # == Bi-directional associations
    #
    # When you specify an association there is usually an association on the associated model that specifies the same
    # relationship in reverse.  For example, with the following models:
    #
    #    class Dungeon < ActiveRecord::Base
    #      has_many :traps
    #      has_one :evil_wizard
    #    end
    #
    #    class Trap < ActiveRecord::Base
    #      belongs_to :dungeon
    #    end
    #
    #    class EvilWizard < ActiveRecord::Base
    #      belongs_to :dungeon
    #    end
    #
    # The +traps+ association on +Dungeon+ and the the +dungeon+ association on +Trap+ are the inverse of each other and the
    # inverse of the +dungeon+ association on +EvilWizard+ is the +evil_wizard+ association on +Dungeon+ (and vice-versa).  By default,
    # Active Record doesn't know anything about these inverse relationships and so no object loading optimisation is possible.  For example:
    #
    #    d = Dungeon.first
    #    t = d.traps.first
    #    d.level == t.dungeon.level # => true
    #    d.level = 10
    #    d.level == t.dungeon.level # => false
    #
    # The +Dungeon+ instances +d+ and <tt>t.dungeon</tt> in the above example refer to the same object data from the database, but are
    # actually different in-memory copies of that data.  Specifying the <tt>:inverse_of</tt> option on associations lets you tell
    # Active Record about inverse relationships and it will optimise object loading.  For example, if we changed our model definitions to:
    #
    #    class Dungeon < ActiveRecord::Base
    #      has_many :traps, :inverse_of => :dungeon
    #      has_one :evil_wizard, :inverse_of => :dungeon
    #    end
    #
    #    class Trap < ActiveRecord::Base
    #      belongs_to :dungeon, :inverse_of => :traps
    #    end
    #
    #    class EvilWizard < ActiveRecord::Base
    #      belongs_to :dungeon, :inverse_of => :evil_wizard
    #    end
    #
    # Then, from our code snippet above, +d+ and <tt>t.dungeon</tt> are actually the same in-memory instance and our final <tt>d.level == t.dungeon.level</tt>
    # will return +true+.
    #
    # There are limitations to <tt>:inverse_of</tt> support:
    #
    # * does not work with <tt>:through</tt> associations.
    # * does not work with <tt>:polymorphic</tt> associations.
    # * for +belongs_to+ associations +has_many+ inverse associations are ignored.
    #
    # == Type safety with <tt>ActiveRecord::AssociationTypeMismatch</tt>
    #
    # If you attempt to assign an object to an association that doesn't match the inferred or specified <tt>:class_name</tt>, you'll
    # get an <tt>ActiveRecord::AssociationTypeMismatch</tt>.
    #
    # == Options
    #
    # All of the association macros can be specialized through options. This makes cases more complex than the simple and guessable ones
    # possible.
    module ClassMethods
      # Specifies a one-to-many association. The following methods for retrieval and query of
      # collections of associated objects will be added:
      #
      # [collection(force_reload = false)]
      #   Returns an array of all the associated objects.
      #   An empty array is returned if none are found.
      # [collection<<(object, ...)]
      #   Adds one or more objects to the collection by setting their foreign keys to the collection's primary key.
      #   Note that this operation instantly fires update sql without waiting for the save or update call on the 
      #   parent object.
      # [collection.delete(object, ...)]
      #   Removes one or more objects from the collection by setting their foreign keys to +NULL+.
      #   Objects will be in addition destroyed if they're associated with <tt>:dependent => :destroy</tt>,
      #   and deleted if they're associated with <tt>:dependent => :delete_all</tt>.
      # [collection=objects]
      #   Replaces the collections content by deleting and adding objects as appropriate. If the <tt>:through</tt>
      #   option is true callbacks in the join models are triggered except destroy callbacks, since deletion is
      #   direct.
      # [collection_singular_ids]
      #   Returns an array of the associated objects' ids
      # [collection_singular_ids=ids]
      #   Replace the collection with the objects identified by the primary keys in +ids+. This
      #   method loads the models and calls <tt>collection=</tt>. See above.
      # [collection.clear]
      #   Removes every object from the collection. This destroys the associated objects if they
      #   are associated with <tt>:dependent => :destroy</tt>, deletes them directly from the
      #   database if <tt>:dependent => :delete_all</tt>, otherwise sets their foreign keys to +NULL+.
      #   If the <tt>:through</tt> option is true no destroy callbacks are invoked on the join models.
      #   Join models are directly deleted.
      # [collection.empty?]
      #   Returns +true+ if there are no associated objects.
      # [collection.size]
      #   Returns the number of associated objects.
      # [collection.find(...)]
      #   Finds an associated object according to the same rules as ActiveRecord::Base.find.
      # [collection.exists?(...)]
      #   Checks whether an associated object with the given conditions exists.
      #   Uses the same rules as ActiveRecord::Base.exists?.
      # [collection.build(attributes = {}, ...)]
      #   Returns one or more new objects of the collection type that have been instantiated
      #   with +attributes+ and linked to this object through a foreign key, but have not yet
      #   been saved.
      # [collection.create(attributes = {})]
      #   Returns a new object of the collection type that has been instantiated
      #   with +attributes+, linked to this object through a foreign key, and that has already
      #   been saved (if it passed the validation). *Note*: This only works if the base model
      #   already exists in the DB, not if it is a new (unsaved) record!
      #
      # (*Note*: +collection+ is replaced with the symbol passed as the first argument, so
      # <tt>has_many :clients</tt> would add among others <tt>clients.empty?</tt>.)
      #
      # === Example
      #
      # Example: A Firm class declares <tt>has_many :clients</tt>, which will add:
      # * <tt>Firm#clients</tt> (similar to <tt>Clients.find :all, :conditions => ["firm_id = ?", id]</tt>)
      # * <tt>Firm#clients<<</tt>
      # * <tt>Firm#clients.delete</tt>
      # * <tt>Firm#clients=</tt>
      # * <tt>Firm#client_ids</tt>
      # * <tt>Firm#client_ids=</tt>
      # * <tt>Firm#clients.clear</tt>
      # * <tt>Firm#clients.empty?</tt> (similar to <tt>firm.clients.size == 0</tt>)
      # * <tt>Firm#clients.size</tt> (similar to <tt>Client.count "firm_id = #{id}"</tt>)
      # * <tt>Firm#clients.find</tt> (similar to <tt>Client.find(id, :conditions => "firm_id = #{id}")</tt>)
      # * <tt>Firm#clients.exists?(:name => 'ACME')</tt> (similar to <tt>Client.exists?(:name => 'ACME', :firm_id => firm.id)</tt>)
      # * <tt>Firm#clients.build</tt> (similar to <tt>Client.new("firm_id" => id)</tt>)
      # * <tt>Firm#clients.create</tt> (similar to <tt>c = Client.new("firm_id" => id); c.save; c</tt>)
      # The declaration can also include an options hash to specialize the behavior of the association.
      #
      # === Supported options
      # [:class_name]
      #   Specify the class name of the association. Use it only if that name can't be inferred
      #   from the association name. So <tt>has_many :products</tt> will by default be linked to the Product class, but
      #   if the real class name is SpecialProduct, you'll have to specify it with this option.
      # [:conditions]
      #   Specify the conditions that the associated objects must meet in order to be included as a +WHERE+
      #   SQL fragment, such as <tt>price > 5 AND name LIKE 'B%'</tt>.  Record creations from the association are scoped if a hash
      #   is used.  <tt>has_many :posts, :conditions => {:published => true}</tt> will create published posts with <tt>@blog.posts.create</tt>
      #   or <tt>@blog.posts.build</tt>.
      # [:order]
      #   Specify the order in which the associated objects are returned as an <tt>ORDER BY</tt> SQL fragment,
      #   such as <tt>last_name, first_name DESC</tt>.
      # [:foreign_key]
      #   Specify the foreign key used for the association. By default this is guessed to be the name
      #   of this class in lower-case and "_id" suffixed. So a Person class that makes a +has_many+ association will use "person_id"
      #   as the default <tt>:foreign_key</tt>.
      # [:primary_key]
      #   Specify the method that returns the primary key used for the association. By default this is +id+.
      # [:dependent]
      #   If set to <tt>:destroy</tt> all the associated objects are destroyed
      #   alongside this object by calling their +destroy+ method.  If set to <tt>:delete_all</tt> all associated
      #   objects are deleted *without* calling their +destroy+ method.  If set to <tt>:nullify</tt> all associated
      #   objects' foreign keys are set to +NULL+ *without* calling their +save+ callbacks. If set to
      #   <tt>:restrict</tt> this object cannot be deleted if it has any associated object.
      #
      #   *Warning:* This option is ignored when used with <tt>:through</tt> option.
      #
      # [:finder_sql]
      #   Specify a complete SQL statement to fetch the association. This is a good way to go for complex
      #   associations that depend on multiple tables. Note: When this option is used, +find_in_collection+ is _not_ added.
      # [:counter_sql]
      #   Specify a complete SQL statement to fetch the size of the association. If <tt>:finder_sql</tt> is
      #   specified but not <tt>:counter_sql</tt>, <tt>:counter_sql</tt> will be generated by replacing <tt>SELECT ... FROM</tt> with <tt>SELECT COUNT(*) FROM</tt>.
      # [:extend]
      #   Specify a named module for extending the proxy. See "Association extensions".
      # [:include]
      #   Specify second-order associations that should be eager loaded when the collection is loaded.
      # [:group]
      #   An attribute name by which the result should be grouped. Uses the <tt>GROUP BY</tt> SQL-clause.
      # [:having]
      #   Combined with +:group+ this can be used to filter the records that a <tt>GROUP BY</tt> returns. Uses the <tt>HAVING</tt> SQL-clause.
      # [:limit]
      #   An integer determining the limit on the number of rows that should be returned.
      # [:offset]
      #   An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
      # [:select]
      #   By default, this is <tt>*</tt> as in <tt>SELECT * FROM</tt>, but can be changed if you, for example, want to do a join
      #   but not include the joined columns. Do not forget to include the primary and foreign keys, otherwise it will raise an error.
      # [:as]
      #   Specifies a polymorphic interface (See <tt>belongs_to</tt>).
      # [:through]
      #   Specifies a join model through which to perform the query.  Options for <tt>:class_name</tt> and <tt>:foreign_key</tt>
      #   are ignored, as the association uses the source reflection. You can only use a <tt>:through</tt> query through a <tt>belongs_to</tt>
      #   <tt>has_one</tt> or <tt>has_many</tt> association on the join model. The collection of join models can be managed via the collection
      #   API. For example, new join models are created for newly associated objects, and if some are gone their rows are deleted (directly,
      #   no destroy callbacks are triggered).
      # [:source]
      #   Specifies the source association name used by <tt>has_many :through</tt> queries.  Only use it if the name cannot be
      #   inferred from the association.  <tt>has_many :subscribers, :through => :subscriptions</tt> will look for either <tt>:subscribers</tt> or
      #   <tt>:subscriber</tt> on Subscription, unless a <tt>:source</tt> is given.
      # [:source_type]
      #   Specifies type of the source association used by <tt>has_many :through</tt> queries where the source
      #   association is a polymorphic +belongs_to+.
      # [:uniq]
      #   If true, duplicates will be omitted from the collection. Useful in conjunction with <tt>:through</tt>.
      # [:readonly]
      #   If true, all the associated objects are readonly through the association.
      # [:validate]
      #   If false, don't validate the associated objects when saving the parent object. true by default.
      # [:autosave]
      #   If true, always save the associated objects or destroy them if marked for destruction, when saving the parent object.
      #   If false, never save or destroy the associated objects.
      #   By default, only save associated objects that are new records.
      # [:inverse_of]
      #   Specifies the name of the <tt>belongs_to</tt> association on the associated object that is the inverse of this <tt>has_many</tt>
      #   association.  Does not work in combination with <tt>:through</tt> or <tt>:as</tt> options.
      #   See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail.
      #
      # Option examples:
      #   has_many :comments, :order => "posted_on"
      #   has_many :comments, :include => :author
      #   has_many :people, :class_name => "Person", :conditions => "deleted = 0", :order => "name"
      #   has_many :tracks, :order => "position", :dependent => :destroy
      #   has_many :comments, :dependent => :nullify
      #   has_many :tags, :as => :taggable
      #   has_many :reports, :readonly => true
      #   has_many :subscribers, :through => :subscriptions, :source => :user
      #   has_many :subscribers, :class_name => "Person", :finder_sql =>
      #       'SELECT DISTINCT people.* ' +
      #       'FROM people p, post_subscriptions ps ' +
      #       'WHERE ps.post_id = #{id} AND ps.person_id = p.id ' +
      #       'ORDER BY p.first_name'
      def has_many(association_id, options = {}, &extension)
        reflection = create_has_many_reflection(association_id, options, &extension)
        configure_dependency_for_has_many(reflection)
        add_association_callbacks(reflection.name, reflection.options)

        if options[:through]
          collection_accessor_methods(reflection, HasManyThroughAssociation)
        else
          collection_accessor_methods(reflection, HasManyAssociation)
        end
      end

      # Specifies a one-to-one association with another class. This method should only be used
      # if the other class contains the foreign key. If the current class contains the foreign key,
      # then you should use +belongs_to+ instead. See also ActiveRecord::Associations::ClassMethods's overview
      # on when to use has_one and when to use belongs_to.
      #
      # The following methods for retrieval and query of a single associated object will be added:
      #
      # [association(force_reload = false)]
      #   Returns the associated object. +nil+ is returned if none is found.
      # [association=(associate)]
      #   Assigns the associate object, extracts the primary key, sets it as the foreign key,
      #   and saves the associate object.
      # [build_association(attributes = {})]
      #   Returns a new object of the associated type that has been instantiated
      #   with +attributes+ and linked to this object through a foreign key, but has not
      #   yet been saved. <b>Note:</b> This ONLY works if an association already exists.
      #   It will NOT work if the association is +nil+.
      # [create_association(attributes = {})]
      #   Returns a new object of the associated type that has been instantiated
      #   with +attributes+, linked to this object through a foreign key, and that
      #   has already been saved (if it passed the validation).
      #
      # (+association+ is replaced with the symbol passed as the first argument, so
      # <tt>has_one :manager</tt> would add among others <tt>manager.nil?</tt>.)
      #
      # === Example
      #
      # An Account class declares <tt>has_one :beneficiary</tt>, which will add:
      # * <tt>Account#beneficiary</tt> (similar to <tt>Beneficiary.find(:first, :conditions => "account_id = #{id}")</tt>)
      # * <tt>Account#beneficiary=(beneficiary)</tt> (similar to <tt>beneficiary.account_id = account.id; beneficiary.save</tt>)
      # * <tt>Account#build_beneficiary</tt> (similar to <tt>Beneficiary.new("account_id" => id)</tt>)
      # * <tt>Account#create_beneficiary</tt> (similar to <tt>b = Beneficiary.new("account_id" => id); b.save; b</tt>)
      #
      # === Options
      #
      # The declaration can also include an options hash to specialize the behavior of the association.
      #
      # Options are:
      # [:class_name]
      #   Specify the class name of the association. Use it only if that name can't be inferred
      #   from the association name. So <tt>has_one :manager</tt> will by default be linked to the Manager class, but
      #   if the real class name is Person, you'll have to specify it with this option.
      # [:conditions]
      #   Specify the conditions that the associated object must meet in order to be included as a +WHERE+
      #   SQL fragment, such as <tt>rank = 5</tt>. Record creation from the association is scoped if a hash
      #   is used. <tt>has_one :account, :conditions => {:enabled => true}</tt> will create an enabled account with <tt>@company.create_account</tt>
      #   or <tt>@company.build_account</tt>.
      # [:order]
      #   Specify the order in which the associated objects are returned as an <tt>ORDER BY</tt> SQL fragment,
      #   such as <tt>last_name, first_name DESC</tt>.
      # [:dependent]
      #   If set to <tt>:destroy</tt>, the associated object is destroyed when this object is. If set to
      #   <tt>:delete</tt>, the associated object is deleted *without* calling its destroy method. If set to <tt>:nullify</tt>, the associated
      #   object's foreign key is set to +NULL+. Also, association is assigned.
      # [:foreign_key]
      #   Specify the foreign key used for the association. By default this is guessed to be the name
      #   of this class in lower-case and "_id" suffixed. So a Person class that makes a +has_one+ association will use "person_id"
      #   as the default <tt>:foreign_key</tt>.
      # [:primary_key]
      #   Specify the method that returns the primary key used for the association. By default this is +id+.
      # [:include]
      #   Specify second-order associations that should be eager loaded when this object is loaded.
      # [:as]
      #   Specifies a polymorphic interface (See <tt>belongs_to</tt>).
      # [:select]
      #   By default, this is <tt>*</tt> as in <tt>SELECT * FROM</tt>, but can be changed if, for example, you want to do a join
      #   but not include the joined columns. Do not forget to include the primary and foreign keys, otherwise it will raise an error.
      # [:through]
      #   Specifies a Join Model through which to perform the query.  Options for <tt>:class_name</tt> and <tt>:foreign_key</tt>
      #   are ignored, as the association uses the source reflection. You can only use a <tt>:through</tt> query through a
      #   <tt>has_one</tt> or <tt>belongs_to</tt> association on the join model.
      # [:source]
      #   Specifies the source association name used by <tt>has_one :through</tt> queries.  Only use it if the name cannot be
      #   inferred from the association.  <tt>has_one :favorite, :through => :favorites</tt> will look for a
      #   <tt>:favorite</tt> on Favorite, unless a <tt>:source</tt> is given.
      # [:source_type]
      #   Specifies type of the source association used by <tt>has_one :through</tt> queries where the source
      #   association is a polymorphic +belongs_to+.
      # [:readonly]
      #   If true, the associated object is readonly through the association.
      # [:validate]
      #   If false, don't validate the associated object when saving the parent object. +false+ by default.
      # [:autosave]
      #   If true, always save the associated object or destroy it if marked for destruction, when saving the parent object.
      #   If false, never save or destroy the associated object.
      #   By default, only save the associated object if it's a new record.
      # [:inverse_of]
      #   Specifies the name of the <tt>belongs_to</tt> association on the associated object that is the inverse of this <tt>has_one</tt>
      #   association.  Does not work in combination with <tt>:through</tt> or <tt>:as</tt> options.
      #   See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail.
      #
      # Option examples:
      #   has_one :credit_card, :dependent => :destroy  # destroys the associated credit card
      #   has_one :credit_card, :dependent => :nullify  # updates the associated records foreign key value to NULL rather than destroying it
      #   has_one :last_comment, :class_name => "Comment", :order => "posted_on"
      #   has_one :project_manager, :class_name => "Person", :conditions => "role = 'project_manager'"
      #   has_one :attachment, :as => :attachable
      #   has_one :boss, :readonly => :true
      #   has_one :club, :through => :membership
      #   has_one :primary_address, :through => :addressables, :conditions => ["addressable.primary = ?", true], :source => :addressable
      def has_one(association_id, options = {})
        if options[:through]
          reflection = create_has_one_through_reflection(association_id, options)
          association_accessor_methods(reflection, ActiveRecord::Associations::HasOneThroughAssociation)
        else
          reflection = create_has_one_reflection(association_id, options)
          association_accessor_methods(reflection, HasOneAssociation)
          association_constructor_method(:build,  reflection, HasOneAssociation)
          association_constructor_method(:create, reflection, HasOneAssociation)
          configure_dependency_for_has_one(reflection)
        end
      end

      # Specifies a one-to-one association with another class. This method should only be used
      # if this class contains the foreign key. If the other class contains the foreign key,
      # then you should use +has_one+ instead. See also ActiveRecord::Associations::ClassMethods's overview
      # on when to use +has_one+ and when to use +belongs_to+.
      #
      # Methods will be added for retrieval and query for a single associated object, for which
      # this object holds an id:
      #
      # [association(force_reload = false)]
      #   Returns the associated object. +nil+ is returned if none is found.
      # [association=(associate)]
      #   Assigns the associate object, extracts the primary key, and sets it as the foreign key.
      # [build_association(attributes = {})]
      #   Returns a new object of the associated type that has been instantiated
      #   with +attributes+ and linked to this object through a foreign key, but has not yet been saved.
      # [create_association(attributes = {})]
      #   Returns a new object of the associated type that has been instantiated
      #   with +attributes+, linked to this object through a foreign key, and that
      #   has already been saved (if it passed the validation).
      #
      # (+association+ is replaced with the symbol passed as the first argument, so
      # <tt>belongs_to :author</tt> would add among others <tt>author.nil?</tt>.)
      #
      # === Example
      #
      # A Post class declares <tt>belongs_to :author</tt>, which will add:
      # * <tt>Post#author</tt> (similar to <tt>Author.find(author_id)</tt>)
      # * <tt>Post#author=(author)</tt> (similar to <tt>post.author_id = author.id</tt>)
      # * <tt>Post#build_author</tt> (similar to <tt>post.author = Author.new</tt>)
      # * <tt>Post#create_author</tt> (similar to <tt>post.author = Author.new; post.author.save; post.author</tt>)
      # The declaration can also include an options hash to specialize the behavior of the association.
      #
      # === Options
      #
      # [:class_name]
      #   Specify the class name of the association. Use it only if that name can't be inferred
      #   from the association name. So <tt>has_one :author</tt> will by default be linked to the Author class, but
      #   if the real class name is Person, you'll have to specify it with this option.
      # [:conditions]
      #   Specify the conditions that the associated object must meet in order to be included as a +WHERE+
      #   SQL fragment, such as <tt>authorized = 1</tt>.
      # [:select]
      #   By default, this is <tt>*</tt> as in <tt>SELECT * FROM</tt>, but can be changed if, for example, you want to do a join
      #   but not include the joined columns. Do not forget to include the primary and foreign keys, otherwise it will raise an error.
      # [:foreign_key]
      #   Specify the foreign key used for the association. By default this is guessed to be the name
      #   of the association with an "_id" suffix. So a class that defines a <tt>belongs_to :person</tt> association will use
      #   "person_id" as the default <tt>:foreign_key</tt>. Similarly, <tt>belongs_to :favorite_person, :class_name => "Person"</tt>
      #   will use a foreign key of "favorite_person_id".
      # [:primary_key]
      #   Specify the method that returns the primary key of associated object used for the association. By default this is id.
      # [:dependent]
      #   If set to <tt>:destroy</tt>, the associated object is destroyed when this object is. If set to
      #   <tt>:delete</tt>, the associated object is deleted *without* calling its destroy method. This option should not be specified when
      #   <tt>belongs_to</tt> is used in conjunction with a <tt>has_many</tt> relationship on another class because of the potential to leave
      #   orphaned records behind.
      # [:counter_cache]
      #   Caches the number of belonging objects on the associate class through the use of +increment_counter+
      #   and +decrement_counter+. The counter cache is incremented when an object of this class is created and decremented when it's
      #   destroyed. This requires that a column named <tt>#{table_name}_count</tt> (such as +comments_count+ for a belonging Comment class)
      #   is used on the associate class (such as a Post class). You can also specify a custom counter cache column by providing
      #   a column name instead of a +true+/+false+ value to this option (e.g., <tt>:counter_cache => :my_custom_counter</tt>.)
      #   Note: Specifying a counter cache will add it to that model's list of readonly attributes using +attr_readonly+.
      # [:include]
      #   Specify second-order associations that should be eager loaded when this object is loaded.
      # [:polymorphic]
      #   Specify this association is a polymorphic association by passing +true+.
      #   Note: If you've enabled the counter cache, then you may want to add the counter cache attribute
      #   to the +attr_readonly+ list in the associated classes (e.g. <tt>class Post; attr_readonly :comments_count; end</tt>).
      # [:readonly]
      #   If true, the associated object is readonly through the association.
      # [:validate]
      #   If false, don't validate the associated objects when saving the parent object. +false+ by default.
      # [:autosave]
      #   If true, always save the associated object or destroy it if marked for destruction, when saving the parent object.
      #   If false, never save or destroy the associated object.
      #   By default, only save the associated object if it's a new record.
      # [:touch]
      #   If true, the associated object will be touched (the updated_at/on attributes set to now) when this record is either saved or
      #   destroyed. If you specify a symbol, that attribute will be updated with the current time instead of the updated_at/on attribute.
      # [:inverse_of]
      #   Specifies the name of the <tt>has_one</tt> or <tt>has_many</tt> association on the associated object that is the inverse of this <tt>belongs_to</tt>
      #   association.  Does not work in combination with the <tt>:polymorphic</tt> options.
      #   See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail.
      #
      # Option examples:
      #   belongs_to :firm, :foreign_key => "client_of"
      #   belongs_to :person, :primary_key => "name", :foreign_key => "person_name"
      #   belongs_to :author, :class_name => "Person", :foreign_key => "author_id"
      #   belongs_to :valid_coupon, :class_name => "Coupon", :foreign_key => "coupon_id",
      #              :conditions => 'discounts > #{payments_count}'
      #   belongs_to :attachable, :polymorphic => true
      #   belongs_to :project, :readonly => true
      #   belongs_to :post, :counter_cache => true
      #   belongs_to :company, :touch => true
      #   belongs_to :company, :touch => :employees_last_updated_at
      def belongs_to(association_id, options = {})
        reflection = create_belongs_to_reflection(association_id, options)

        if reflection.options[:polymorphic]
          association_accessor_methods(reflection, BelongsToPolymorphicAssociation)
        else
          association_accessor_methods(reflection, BelongsToAssociation)
          association_constructor_method(:build,  reflection, BelongsToAssociation)
          association_constructor_method(:create, reflection, BelongsToAssociation)
        end

        add_counter_cache_callbacks(reflection)          if options[:counter_cache]
        add_touch_callbacks(reflection, options[:touch]) if options[:touch]

        configure_dependency_for_belongs_to(reflection)
      end

      # Specifies a many-to-many relationship with another class. This associates two classes via an
      # intermediate join table.  Unless the join table is explicitly specified as an option, it is
      # guessed using the lexical order of the class names. So a join between Developer and Project
      # will give the default join table name of "developers_projects" because "D" outranks "P".  Note that this precedence
      # is calculated using the <tt><</tt> operator for String.  This means that if the strings are of different lengths,
      # and the strings are equal when compared up to the shortest length, then the longer string is considered of higher
      # lexical precedence than the shorter one.  For example, one would expect the tables "paper_boxes" and "papers"
      # to generate a join table name of "papers_paper_boxes" because of the length of the name "paper_boxes",
      # but it in fact generates a join table name of "paper_boxes_papers".  Be aware of this caveat, and use the
      # custom <tt>:join_table</tt> option if you need to.
      #
      # The join table should not have a primary key or a model associated with it. You must manually generate the
      # join table with a migration such as this:
      #
      #   class CreateDevelopersProjectsJoinTable < ActiveRecord::Migration
      #     def self.up
      #       create_table :developers_projects, :id => false do |t|
      #         t.integer :developer_id
      #         t.integer :project_id
      #       end
      #     end
      #
      #     def self.down
      #       drop_table :developers_projects
      #     end
      #   end
      #
      # Deprecated: Any additional fields added to the join table will be placed as attributes when pulling records out through
      # +has_and_belongs_to_many+ associations. Records returned from join tables with additional attributes will be marked as
      # readonly (because we can't save changes to the additional attributes). It's strongly recommended that you upgrade any
      # associations with attributes to a real join model (see introduction).
      #
      # Adds the following methods for retrieval and query:
      #
      # [collection(force_reload = false)]
      #   Returns an array of all the associated objects.
      #   An empty array is returned if none are found.
      # [collection<<(object, ...)]
      #   Adds one or more objects to the collection by creating associations in the join table
      #   (<tt>collection.push</tt> and <tt>collection.concat</tt> are aliases to this method).
      #   Note that this operation instantly fires update sql without waiting for the save or update call on the 
      #   parent object.
      # [collection.delete(object, ...)]
      #   Removes one or more objects from the collection by removing their associations from the join table.
      #   This does not destroy the objects.
      # [collection=objects]
      #   Replaces the collection's content by deleting and adding objects as appropriate.
      # [collection_singular_ids]
      #   Returns an array of the associated objects' ids.
      # [collection_singular_ids=ids]
      #   Replace the collection by the objects identified by the primary keys in +ids+.
      # [collection.clear]
      #   Removes every object from the collection. This does not destroy the objects.
      # [collection.empty?]
      #   Returns +true+ if there are no associated objects.
      # [collection.size]
      #   Returns the number of associated objects.
      # [collection.find(id)]
      #   Finds an associated object responding to the +id+ and that
      #   meets the condition that it has to be associated with this object.
      #   Uses the same rules as ActiveRecord::Base.find.
      # [collection.exists?(...)]
      #   Checks whether an associated object with the given conditions exists.
      #   Uses the same rules as ActiveRecord::Base.exists?.
      # [collection.build(attributes = {})]
      #   Returns a new object of the collection type that has been instantiated
      #   with +attributes+ and linked to this object through the join table, but has not yet been saved.
      # [collection.create(attributes = {})]
      #   Returns a new object of the collection type that has been instantiated
      #   with +attributes+, linked to this object through the join table, and that has already been saved (if it passed the validation).
      #
      # (+collection+ is replaced with the symbol passed as the first argument, so
      # <tt>has_and_belongs_to_many :categories</tt> would add among others <tt>categories.empty?</tt>.)
      #
      # === Example
      #
      # A Developer class declares <tt>has_and_belongs_to_many :projects</tt>, which will add:
      # * <tt>Developer#projects</tt>
      # * <tt>Developer#projects<<</tt>
      # * <tt>Developer#projects.delete</tt>
      # * <tt>Developer#projects=</tt>
      # * <tt>Developer#project_ids</tt>
      # * <tt>Developer#project_ids=</tt>
      # * <tt>Developer#projects.clear</tt>
      # * <tt>Developer#projects.empty?</tt>
      # * <tt>Developer#projects.size</tt>
      # * <tt>Developer#projects.find(id)</tt>
      # * <tt>Developer#projects.exists?(...)</tt>
      # * <tt>Developer#projects.build</tt> (similar to <tt>Project.new("project_id" => id)</tt>)
      # * <tt>Developer#projects.create</tt> (similar to <tt>c = Project.new("project_id" => id); c.save; c</tt>)
      # The declaration may include an options hash to specialize the behavior of the association.
      #
      # === Options
      #
      # [:class_name]
      #   Specify the class name of the association. Use it only if that name can't be inferred
      #   from the association name. So <tt>has_and_belongs_to_many :projects</tt> will by default be linked to the
      #   Project class, but if the real class name is SuperProject, you'll have to specify it with this option.
      # [:join_table]
      #   Specify the name of the join table if the default based on lexical order isn't what you want.
      #   <b>WARNING:</b> If you're overwriting the table name of either class, the +table_name+ method
      #   MUST be declared underneath any +has_and_belongs_to_many+ declaration in order to work.
      # [:foreign_key]
      #   Specify the foreign key used for the association. By default this is guessed to be the name
      #   of this class in lower-case and "_id" suffixed. So a Person class that makes a +has_and_belongs_to_many+ association
      #   to Project will use "person_id" as the default <tt>:foreign_key</tt>.
      # [:association_foreign_key]
      #   Specify the foreign key used for the association on the receiving side of the association.
      #   By default this is guessed to be the name of the associated class in lower-case and "_id" suffixed.
      #   So if a Person class makes a +has_and_belongs_to_many+ association to Project,
      #   the association will use "project_id" as the default <tt>:association_foreign_key</tt>.
      # [:conditions]
      #   Specify the conditions that the associated object must meet in order to be included as a +WHERE+
      #   SQL fragment, such as <tt>authorized = 1</tt>.  Record creations from the association are scoped if a hash is used.
      #   <tt>has_many :posts, :conditions => {:published => true}</tt> will create published posts with <tt>@blog.posts.create</tt>
      #   or <tt>@blog.posts.build</tt>.
      # [:order]
      #   Specify the order in which the associated objects are returned as an <tt>ORDER BY</tt> SQL fragment,
      #   such as <tt>last_name, first_name DESC</tt>
      # [:uniq]
      #   If true, duplicate associated objects will be ignored by accessors and query methods.
      # [:finder_sql]
      #   Overwrite the default generated SQL statement used to fetch the association with a manual statement
      # [:counter_sql]
      #   Specify a complete SQL statement to fetch the size of the association. If <tt>:finder_sql</tt> is
      #   specified but not <tt>:counter_sql</tt>, <tt>:counter_sql</tt> will be generated by replacing <tt>SELECT ... FROM</tt> with <tt>SELECT COUNT(*) FROM</tt>.
      # [:delete_sql]
      #   Overwrite the default generated SQL statement used to remove links between the associated
      #   classes with a manual statement.
      # [:insert_sql]
      #   Overwrite the default generated SQL statement used to add links between the associated classes
      #   with a manual statement.
      # [:extend]
      #   Anonymous module for extending the proxy, see "Association extensions".
      # [:include]
      #   Specify second-order associations that should be eager loaded when the collection is loaded.
      # [:group]
      #   An attribute name by which the result should be grouped. Uses the <tt>GROUP BY</tt> SQL-clause.
      # [:having]
      #   Combined with +:group+ this can be used to filter the records that a <tt>GROUP BY</tt> returns. Uses the <tt>HAVING</tt> SQL-clause.
      # [:limit]
      #   An integer determining the limit on the number of rows that should be returned.
      # [:offset]
      #   An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
      # [:select]
      #   By default, this is <tt>*</tt> as in <tt>SELECT * FROM</tt>, but can be changed if, for example, you want to do a join
      #   but not include the joined columns. Do not forget to include the primary and foreign keys, otherwise it will raise an error.
      # [:readonly]
      #   If true, all the associated objects are readonly through the association.
      # [:validate]
      #   If false, don't validate the associated objects when saving the parent object. +true+ by default.
      # [:autosave]
      #   If true, always save the associated objects or destroy them if marked for destruction, when saving the parent object.
      #   If false, never save or destroy the associated objects.
      #   By default, only save associated objects that are new records.
      #
      # Option examples:
      #   has_and_belongs_to_many :projects
      #   has_and_belongs_to_many :projects, :include => [ :milestones, :manager ]
      #   has_and_belongs_to_many :nations, :class_name => "Country"
      #   has_and_belongs_to_many :categories, :join_table => "prods_cats"
      #   has_and_belongs_to_many :categories, :readonly => true
      #   has_and_belongs_to_many :active_projects, :join_table => 'developers_projects', :delete_sql =>
      #   'DELETE FROM developers_projects WHERE active=1 AND developer_id = #{id} AND project_id = #{record.id}'
      def has_and_belongs_to_many(association_id, options = {}, &extension)
        reflection = create_has_and_belongs_to_many_reflection(association_id, options, &extension)
        collection_accessor_methods(reflection, HasAndBelongsToManyAssociation)

        # Don't use a before_destroy callback since users' before_destroy
        # callbacks will be executed after the association is wiped out.
        include Module.new {
          class_eval <<-RUBY, __FILE__, __LINE__ + 1
            def destroy                     # def destroy
              super                         #   super
              #{reflection.name}.clear      #   posts.clear
            end                             # end
          RUBY
        }

        add_association_callbacks(reflection.name, options)
      end

      private
        # Generates a join table name from two provided table names.
        # The names in the join table names end up in lexicographic order.
        #
        #   join_table_name("members", "clubs")         # => "clubs_members"
        #   join_table_name("members", "special_clubs") # => "members_special_clubs"
        def join_table_name(first_table_name, second_table_name)
          if first_table_name < second_table_name
            join_table = "#{first_table_name}_#{second_table_name}"
          else
            join_table = "#{second_table_name}_#{first_table_name}"
          end

          table_name_prefix + join_table + table_name_suffix
        end

        def association_accessor_methods(reflection, association_proxy_class)
          redefine_method(reflection.name) do |*params|
            force_reload = params.first unless params.empty?
            association = association_instance_get(reflection.name)

            if association.nil? || force_reload
              association = association_proxy_class.new(self, reflection)
              retval = force_reload ? reflection.klass.uncached { association.reload } : association.reload
              if retval.nil? and association_proxy_class == BelongsToAssociation
                association_instance_set(reflection.name, nil)
                return nil
              end
              association_instance_set(reflection.name, association)
            end

            association.target.nil? ? nil : association
          end

          redefine_method("loaded_#{reflection.name}?") do
            association = association_instance_get(reflection.name)
            association && association.loaded?
          end
          
          redefine_method("#{reflection.name}=") do |new_value|
            association = association_instance_get(reflection.name)

            if association.nil? || association.target != new_value
              association = association_proxy_class.new(self, reflection)
            end

            association.replace(new_value)
            association_instance_set(reflection.name, new_value.nil? ? nil : association)
          end
          
          redefine_method("set_#{reflection.name}_target") do |target|
            return if target.nil? and association_proxy_class == BelongsToAssociation
            association = association_proxy_class.new(self, reflection)
            association.target = target
            association_instance_set(reflection.name, association)
          end
        end

        def collection_reader_method(reflection, association_proxy_class)
          redefine_method(reflection.name) do |*params|
            force_reload = params.first unless params.empty?
            association = association_instance_get(reflection.name)

            unless association
              association = association_proxy_class.new(self, reflection)
              association_instance_set(reflection.name, association)
            end

            reflection.klass.uncached { association.reload } if force_reload

            association
          end
          
          redefine_method("#{reflection.name.to_s.singularize}_ids") do
            if send(reflection.name).loaded? || reflection.options[:finder_sql]
              send(reflection.name).map(&:id)
            else
              if reflection.through_reflection && reflection.source_reflection.belongs_to?
                through = reflection.through_reflection
                primary_key = reflection.source_reflection.primary_key_name
                send(through.name).select("DISTINCT #{through.quoted_table_name}.#{primary_key}").map!(&:"#{primary_key}")
              else
                send(reflection.name).select("#{reflection.quoted_table_name}.#{reflection.klass.primary_key}").except(:includes).map!(&:id)
              end
            end
          end

        end

        def collection_accessor_methods(reflection, association_proxy_class, writer = true)
          collection_reader_method(reflection, association_proxy_class)

          if writer
            redefine_method("#{reflection.name}=") do |new_value|
              # Loads proxy class instance (defined in collection_reader_method) if not already loaded
              association = send(reflection.name)
              association.replace(new_value)
              association
            end
            
            redefine_method("#{reflection.name.to_s.singularize}_ids=") do |new_value|
              ids = (new_value || []).reject { |nid| nid.blank? }.map(&:to_i)
              send("#{reflection.name}=", reflection.klass.find(ids).index_by(&:id).values_at(*ids))
            end
          end
        end

        def association_constructor_method(constructor, reflection, association_proxy_class)
          redefine_method("#{constructor}_#{reflection.name}") do |*params|
            attributees      = params.first unless params.empty?
            replace_existing = params[1].nil? ? true : params[1]
            association      = association_instance_get(reflection.name)

            unless association
              association = association_proxy_class.new(self, reflection)
              association_instance_set(reflection.name, association)
            end

            if association_proxy_class == HasOneAssociation
              association.send(constructor, attributees, replace_existing)
            else
              association.send(constructor, attributees)
            end
          end
        end

        def add_counter_cache_callbacks(reflection)
          cache_column = reflection.counter_cache_column

          method_name = "belongs_to_counter_cache_after_create_for_#{reflection.name}".to_sym
          define_method(method_name) do
            association = send(reflection.name)
            association.class.increment_counter(cache_column, association.id) unless association.nil?
          end
          after_create(method_name)

          method_name = "belongs_to_counter_cache_before_destroy_for_#{reflection.name}".to_sym
          define_method(method_name) do
            association = send(reflection.name)
            association.class.decrement_counter(cache_column, association.id) unless association.nil?
          end
          before_destroy(method_name)

          module_eval(
            "#{reflection.class_name}.send(:attr_readonly,\"#{cache_column}\".intern) if defined?(#{reflection.class_name}) && #{reflection.class_name}.respond_to?(:attr_readonly)", __FILE__, __LINE__
          )
        end

        def add_touch_callbacks(reflection, touch_attribute)
          method_name = :"belongs_to_touch_after_save_or_destroy_for_#{reflection.name}"
          redefine_method(method_name) do
            association = send(reflection.name)

            if touch_attribute == true
              association.touch unless association.nil?
            else
              association.touch(touch_attribute) unless association.nil?
            end
          end
          after_save(method_name)
          after_destroy(method_name)
        end

        # Creates before_destroy callback methods that nullify, delete or destroy
        # has_many associated objects, according to the defined :dependent rule.
        # If the association is marked as :dependent => :restrict, create a callback
        # that prevents deleting entirely.
        #
        # See HasManyAssociation#delete_records.  Dependent associations
        # delete children, otherwise foreign key is set to NULL.
        # See HasManyAssociation#delete_records. Dependent associations
        # delete children if the option is set to :destroy or :delete_all, set the
        # foreign key to NULL if the option is set to :nullify, and do not touch the
        # child records if the option is set to :restrict.
        #
        # The +extra_conditions+ parameter, which is not used within the main
        # Active Record codebase, is meant to allow plugins to define extra
        # finder conditions.
        def configure_dependency_for_has_many(reflection, extra_conditions = nil)
          if reflection.options.include?(:dependent)
            case reflection.options[:dependent]
              when :destroy
                method_name = "has_many_dependent_destroy_for_#{reflection.name}".to_sym
                define_method(method_name) do
                  send(reflection.name).each do |o|
                    # No point in executing the counter update since we're going to destroy the parent anyway
                    counter_method = ('belongs_to_counter_cache_before_destroy_for_' + self.class.name.downcase).to_sym
                    if(o.respond_to? counter_method) then
                      class << o
                        self
                      end.send(:define_method, counter_method, Proc.new {})
                    end
                    o.destroy
                  end
                end
                before_destroy method_name
              when :delete_all
                before_destroy do |record|
                  self.class.send(:delete_all_has_many_dependencies,
                  record,
                  reflection.name,
                  reflection.klass,
                  reflection.dependent_conditions(record, self.class, extra_conditions))
                end
              when :nullify
                before_destroy do |record|
                  self.class.send(:nullify_has_many_dependencies,
                  record,
                  reflection.name,
                  reflection.klass,
                  reflection.primary_key_name,
                  reflection.dependent_conditions(record, self.class, extra_conditions))
                end
              when :restrict
                method_name = "has_many_dependent_restrict_for_#{reflection.name}".to_sym
                define_method(method_name) do
                  unless send(reflection.name).empty?
                    raise DeleteRestrictionError.new(reflection)
                  end
                end
                before_destroy method_name
              else
                raise ArgumentError, "The :dependent option expects either :destroy, :delete_all, :nullify or :restrict (#{reflection.options[:dependent].inspect})"
            end
          end
        end

        # Creates before_destroy callback methods that nullify, delete or destroy
        # has_one associated objects, according to the defined :dependent rule.
        # If the association is marked as :dependent => :restrict, create a callback
        # that prevents deleting entirely.
        def configure_dependency_for_has_one(reflection)
          if reflection.options.include?(:dependent)
            name = reflection.options[:dependent]
            method_name = :"has_one_dependent_#{name}_for_#{reflection.name}"

            case name
              when :destroy, :delete
                class_eval <<-eoruby, __FILE__, __LINE__ + 1
                  def #{method_name}
                    association = #{reflection.name}
                    association.#{name} if association
                  end
                eoruby
              when :nullify
                class_eval <<-eoruby, __FILE__, __LINE__ + 1
                  def #{method_name}
                    association = #{reflection.name}
                    association.update_attribute(#{reflection.primary_key_name.inspect}, nil) if association
                  end
                eoruby
              when :restrict
                method_name = "has_one_dependent_restrict_for_#{reflection.name}".to_sym
                define_method(method_name) do
                  unless send(reflection.name).nil?
                    raise DeleteRestrictionError.new(reflection)
                  end
                end
                before_destroy method_name
              else
                raise ArgumentError, "The :dependent option expects either :destroy, :delete, :nullify or :restrict (#{reflection.options[:dependent].inspect})"
            end

            before_destroy method_name
          end
        end

        def configure_dependency_for_belongs_to(reflection)
          if reflection.options.include?(:dependent)
            name = reflection.options[:dependent]

            unless [:destroy, :delete].include?(name)
              raise ArgumentError, "The :dependent option expects either :destroy or :delete (#{reflection.options[:dependent].inspect})"
            end

            method_name = :"belongs_to_dependent_#{name}_for_#{reflection.name}"
            class_eval <<-eoruby, __FILE__, __LINE__ + 1
              def #{method_name}
                association = #{reflection.name}
                association.#{name} if association
              end
            eoruby
            after_destroy method_name
          end
        end

        def delete_all_has_many_dependencies(record, reflection_name, association_class, dependent_conditions)
          association_class.delete_all(dependent_conditions)
        end

        def nullify_has_many_dependencies(record, reflection_name, association_class, primary_key_name, dependent_conditions)
          association_class.update_all("#{primary_key_name} = NULL", dependent_conditions)
        end

        mattr_accessor :valid_keys_for_has_many_association
        @@valid_keys_for_has_many_association = [
          :class_name, :table_name, :foreign_key, :primary_key,
          :dependent,
          :select, :conditions, :include, :order, :group, :having, :limit, :offset,
          :as, :through, :source, :source_type,
          :uniq,
          :finder_sql, :counter_sql,
          :before_add, :after_add, :before_remove, :after_remove,
          :extend, :readonly,
          :validate, :inverse_of
        ]

        def create_has_many_reflection(association_id, options, &extension)
          options.assert_valid_keys(valid_keys_for_has_many_association)
          options[:extend] = create_extension_modules(association_id, extension, options[:extend])

          create_reflection(:has_many, association_id, options, self)
        end

        mattr_accessor :valid_keys_for_has_one_association
        @@valid_keys_for_has_one_association = [
          :class_name, :foreign_key, :remote, :select, :conditions, :order,
          :include, :dependent, :counter_cache, :extend, :as, :readonly,
          :validate, :primary_key, :inverse_of
        ]

        def create_has_one_reflection(association_id, options)
          options.assert_valid_keys(valid_keys_for_has_one_association)
          create_reflection(:has_one, association_id, options, self)
        end

        def create_has_one_through_reflection(association_id, options)
          options.assert_valid_keys(
            :class_name, :foreign_key, :remote, :select, :conditions, :order, :include, :dependent, :counter_cache, :extend, :as, :through, :source, :source_type, :validate
          )
          create_reflection(:has_one, association_id, options, self)
        end

        mattr_accessor :valid_keys_for_belongs_to_association
        @@valid_keys_for_belongs_to_association = [
          :class_name, :primary_key, :foreign_key, :foreign_type, :remote, :select, :conditions,
          :include, :dependent, :counter_cache, :extend, :polymorphic, :readonly,
          :validate, :touch, :inverse_of
        ]

        def create_belongs_to_reflection(association_id, options)
          options.assert_valid_keys(valid_keys_for_belongs_to_association)
          reflection = create_reflection(:belongs_to, association_id, options, self)

          if options[:polymorphic]
            reflection.options[:foreign_type] ||= reflection.class_name.underscore + "_type"
          end

          reflection
        end

        mattr_accessor :valid_keys_for_has_and_belongs_to_many_association
        @@valid_keys_for_has_and_belongs_to_many_association = [
          :class_name, :table_name, :join_table, :foreign_key, :association_foreign_key,
          :select, :conditions, :include, :order, :group, :having, :limit, :offset,
          :uniq,
          :finder_sql, :counter_sql, :delete_sql, :insert_sql,
          :before_add, :after_add, :before_remove, :after_remove,
          :extend, :readonly,
          :validate
        ]

        def create_has_and_belongs_to_many_reflection(association_id, options, &extension)
          options.assert_valid_keys(valid_keys_for_has_and_belongs_to_many_association)
          options[:extend] = create_extension_modules(association_id, extension, options[:extend])

          reflection = create_reflection(:has_and_belongs_to_many, association_id, options, self)

          if reflection.association_foreign_key == reflection.primary_key_name
            raise HasAndBelongsToManyAssociationForeignKeyNeeded.new(reflection)
          end

          reflection.options[:join_table] ||= join_table_name(undecorated_table_name(self.to_s), undecorated_table_name(reflection.class_name))
          if connection.supports_primary_key? && (connection.primary_key(reflection.options[:join_table]) rescue false)
             raise HasAndBelongsToManyAssociationWithPrimaryKeyError.new(reflection)
          end

          reflection
        end

        def add_association_callbacks(association_name, options)
          callbacks = %w(before_add after_add before_remove after_remove)
          callbacks.each do |callback_name|
            full_callback_name = "#{callback_name}_for_#{association_name}"
            defined_callbacks = options[callback_name.to_sym]
            if options.has_key?(callback_name.to_sym)
              class_inheritable_reader full_callback_name.to_sym
              write_inheritable_attribute(full_callback_name.to_sym, [defined_callbacks].flatten)
            else
              write_inheritable_attribute(full_callback_name.to_sym, [])
            end
          end
        end

        def create_extension_modules(association_id, block_extension, extensions)
          if block_extension
            extension_module_name = "#{self.to_s.demodulize}#{association_id.to_s.camelize}AssociationExtension"

            silence_warnings do
              self.parent.const_set(extension_module_name, Module.new(&block_extension))
            end
            Array.wrap(extensions).push("#{self.parent}::#{extension_module_name}".constantize)
          else
            Array.wrap(extensions)
          end
        end

        class JoinDependency # :nodoc:
          attr_reader :joins, :reflections, :table_aliases

          def initialize(base, associations, joins)
            @joins                 = [JoinBase.new(base, joins)]
            @associations          = associations
            @reflections           = []
            @base_records_hash     = {}
            @base_records_in_order = []
            @table_aliases         = Hash.new { |aliases, table| aliases[table] = 0 }
            @table_aliases[base.table_name] = 1
            build(associations)
          end

          def graft(*associations)
            associations.each do |association|
              join_associations.detect {|a| association == a} ||
              build(association.reflection.name, association.find_parent_in(self) || join_base, association.join_class)
            end
            self
          end

          def join_associations
            @joins[1..-1].to_a
          end

          def join_base
            @joins[0]
          end

          def count_aliases_from_table_joins(name)
            # quoted_name should be downcased as some database adapters (Oracle) return quoted name in uppercase
            quoted_name = join_base.active_record.connection.quote_table_name(name.downcase).downcase
            join_sql = join_base.table_joins.to_s.downcase
            join_sql.blank? ? 0 :
              # Table names
              join_sql.scan(/join(?:\s+\w+)?\s+#{quoted_name}\son/).size +
              # Table aliases
              join_sql.scan(/join(?:\s+\w+)?\s+\S+\s+#{quoted_name}\son/).size
          end

          def instantiate(rows)
            rows.each_with_index do |row, i|
              primary_id = join_base.record_id(row)
              unless @base_records_hash[primary_id]
                @base_records_in_order << (@base_records_hash[primary_id] = join_base.instantiate(row))
              end
              construct(@base_records_hash[primary_id], @associations, join_associations.dup, row)
            end
            remove_duplicate_results!(join_base.active_record, @base_records_in_order, @associations)
            return @base_records_in_order
          end

          def remove_duplicate_results!(base, records, associations)
            case associations
              when Symbol, String
                reflection = base.reflections[associations]
                if reflection && reflection.collection?
                  records.each { |record| record.send(reflection.name).target.uniq! }
                end
              when Array
                associations.each do |association|
                  remove_duplicate_results!(base, records, association)
                end
              when Hash
                associations.keys.each do |name|
                  reflection = base.reflections[name]

                  parent_records = []
                  records.each do |record|
                    if descendant = record.send(reflection.name)
                      if reflection.collection?
                        parent_records.concat descendant.target.uniq
                      else
                        parent_records << descendant
                      end
                    end
                  end

                  remove_duplicate_results!(reflection.klass, parent_records, associations[name]) unless parent_records.empty?
                end
            end
          end

          protected
            def build(associations, parent = nil, join_class = Arel::InnerJoin)
              parent ||= @joins.last
              case associations
                when Symbol, String
                  reflection = parent.reflections[associations.to_s.intern] or
                  raise ConfigurationError, "Association named '#{ associations }' was not found; perhaps you misspelled it?"
                  @reflections << reflection
                  @joins << build_join_association(reflection, parent).with_join_class(join_class)
                when Array
                  associations.each do |association|
                    build(association, parent, join_class)
                  end
                when Hash
                  associations.keys.sort{|a,b|a.to_s<=>b.to_s}.each do |name|
                    build(name, parent, join_class)
                    build(associations[name], nil, join_class)
                  end
                else
                  raise ConfigurationError, associations.inspect
              end
            end

            def build_join_association(reflection, parent)
              JoinAssociation.new(reflection, self, parent)
            end

            def construct(parent, associations, joins, row)
              case associations
                when Symbol, String
                  join = joins.detect{|j| j.reflection.name.to_s == associations.to_s && j.parent_table_name == parent.class.table_name }
                  raise(ConfigurationError, "No such association") if join.nil?

                  joins.delete(join)
                  construct_association(parent, join, row)
                when Array
                  associations.each do |association|
                    construct(parent, association, joins, row)
                  end
                when Hash
                  associations.keys.sort{|a,b|a.to_s<=>b.to_s}.each do |name|
                    join = joins.detect{|j| j.reflection.name.to_s == name.to_s && j.parent_table_name == parent.class.table_name }
                    raise(ConfigurationError, "No such association") if join.nil?

                    association = construct_association(parent, join, row)
                    joins.delete(join)
                    construct(association, associations[name], joins, row) if association
                  end
                else
                  raise ConfigurationError, associations.inspect
              end
            end

            def construct_association(record, join, row)
              case join.reflection.macro
                when :has_many, :has_and_belongs_to_many
                  collection = record.send(join.reflection.name)
                  collection.loaded

                  return nil if record.id.to_s != join.parent.record_id(row).to_s or row[join.aliased_primary_key].nil?
                  association = join.instantiate(row)
                  collection.target.push(association)
                  collection.__send__(:set_inverse_instance, association, record)
                when :has_one
                  return if record.id.to_s != join.parent.record_id(row).to_s
                  return if record.instance_variable_defined?("@#{join.reflection.name}")
                  association = join.instantiate(row) unless row[join.aliased_primary_key].nil?
                  set_target_and_inverse(join, association, record)
                when :belongs_to
                  return if record.id.to_s != join.parent.record_id(row).to_s or row[join.aliased_primary_key].nil?
                  association = join.instantiate(row)
                  set_target_and_inverse(join, association, record)
                else
                  raise ConfigurationError, "unknown macro: #{join.reflection.macro}"
              end
              return association
            end

            def set_target_and_inverse(join, association, record)
              association_proxy = record.send("set_#{join.reflection.name}_target", association)
              association_proxy.__send__(:set_inverse_instance, association, record)
            end

          class JoinBase # :nodoc:
            attr_reader :active_record, :table_joins
            delegate    :table_name, :column_names, :primary_key, :reflections, :sanitize_sql, :arel_engine, :to => :active_record

            def initialize(active_record, joins = nil)
              @active_record = active_record
              @cached_record = {}
              @table_joins   = joins
            end

            def ==(other)
              other.class == self.class &&
              other.active_record == active_record &&
              other.table_joins == table_joins
            end

            def aliased_prefix
              "t0"
            end

            def aliased_primary_key
              "#{aliased_prefix}_r0"
            end

            def aliased_table_name
              active_record.table_name
            end

            def column_names_with_alias
              unless defined?(@column_names_with_alias)
                @column_names_with_alias = []

                ([primary_key] + (column_names - [primary_key])).each_with_index do |column_name, i|
                  @column_names_with_alias << [column_name, "#{aliased_prefix}_r#{i}"]
                end
              end

              @column_names_with_alias
            end

            def extract_record(row)
              column_names_with_alias.inject({}){|record, (cn, an)| record[cn] = row[an]; record}
            end

            def record_id(row)
              row[aliased_primary_key]
            end

            def instantiate(row)
              @cached_record[record_id(row)] ||= active_record.send(:instantiate, extract_record(row))
            end
          end

          class JoinAssociation < JoinBase # :nodoc:
            attr_reader :reflection, :parent, :aliased_table_name, :aliased_prefix, :aliased_join_table_name, :parent_table_name, :join_class
            delegate    :options, :klass, :through_reflection, :source_reflection, :to => :reflection

            def initialize(reflection, join_dependency, parent = nil)
              reflection.check_validity!
              if reflection.options[:polymorphic]
                raise EagerLoadPolymorphicError.new(reflection)
              end

              super(reflection.klass)
              @join_dependency    = join_dependency
              @parent             = parent
              @reflection         = reflection
              @aliased_prefix     = "t#{ join_dependency.joins.size }"
              @parent_table_name  = parent.active_record.table_name
              @aliased_table_name = aliased_table_name_for(table_name)
              @join               = nil
              @join_class         = Arel::InnerJoin

              if reflection.macro == :has_and_belongs_to_many
                @aliased_join_table_name = aliased_table_name_for(reflection.options[:join_table], "_join")
              end

              if [:has_many, :has_one].include?(reflection.macro) && reflection.options[:through]
                @aliased_join_table_name = aliased_table_name_for(reflection.through_reflection.klass.table_name, "_join")
              end
            end

            def ==(other)
              other.class == self.class &&
              other.reflection == reflection &&
              other.parent == parent
            end

            def find_parent_in(other_join_dependency)
              other_join_dependency.joins.detect do |join|
                self.parent == join
              end
            end

            def with_join_class(join_class)
              @join_class = join_class
              self
            end

            def association_join
              return @join if @join

              aliased_table = Arel::Table.new(table_name, :as => @aliased_table_name, :engine => arel_engine)
              parent_table = Arel::Table.new(parent.table_name, :as => parent.aliased_table_name, :engine => arel_engine)

              @join = case reflection.macro
              when :has_and_belongs_to_many
                join_table = Arel::Table.new(options[:join_table], :as => aliased_join_table_name, :engine => arel_engine)
                fk = options[:foreign_key] || reflection.active_record.to_s.foreign_key
                klass_fk = options[:association_foreign_key] || klass.to_s.foreign_key

                [
                  join_table[fk].eq(parent_table[reflection.active_record.primary_key]),
                  aliased_table[klass.primary_key].eq(join_table[klass_fk])
                ]
              when :has_many, :has_one
                if reflection.options[:through]
                  join_table = Arel::Table.new(through_reflection.klass.table_name, :as => aliased_join_table_name, :engine => arel_engine)
                  jt_foreign_key = jt_as_extra = jt_source_extra = jt_sti_extra = nil
                  first_key = second_key = as_extra = nil

                  if through_reflection.options[:as] # has_many :through against a polymorphic join
                    jt_foreign_key = through_reflection.options[:as].to_s + '_id'
                    jt_as_extra = join_table[through_reflection.options[:as].to_s + '_type'].eq(parent.active_record.base_class.name)
                  else
                    jt_foreign_key = through_reflection.primary_key_name
                  end

                  case source_reflection.macro
                  when :has_many
                    if source_reflection.options[:as]
                      first_key   = "#{source_reflection.options[:as]}_id"
                      second_key  = options[:foreign_key] || primary_key
                      as_extra    = aliased_table["#{source_reflection.options[:as]}_type"].eq(source_reflection.active_record.base_class.name)
                    else
                      first_key   = through_reflection.klass.base_class.to_s.foreign_key
                      second_key  = options[:foreign_key] || primary_key
                    end

                    unless through_reflection.klass.descends_from_active_record?
                      jt_sti_extra = join_table[through_reflection.active_record.inheritance_column].eq(through_reflection.klass.sti_name)
                    end
                  when :belongs_to
                    first_key = primary_key
                    if reflection.options[:source_type]
                      second_key = source_reflection.association_foreign_key
                      jt_source_extra = join_table[reflection.source_reflection.options[:foreign_type]].eq(reflection.options[:source_type])
                    else
                      second_key = source_reflection.primary_key_name
                    end
                  end

                  [
                    [parent_table[parent.primary_key].eq(join_table[jt_foreign_key]), jt_as_extra, jt_source_extra, jt_sti_extra].reject{|x| x.blank? },
                    aliased_table[first_key].eq(join_table[second_key])
                  ]
                elsif reflection.options[:as]
                  id_rel = aliased_table["#{reflection.options[:as]}_id"].eq(parent_table[parent.primary_key])
                  type_rel = aliased_table["#{reflection.options[:as]}_type"].eq(parent.active_record.base_class.name)
                  [id_rel, type_rel]
                else
                  foreign_key = options[:foreign_key] || reflection.active_record.name.foreign_key
                  [aliased_table[foreign_key].eq(parent_table[reflection.options[:primary_key] || parent.primary_key])]
                end
              when :belongs_to
                [aliased_table[options[:primary_key] || reflection.klass.primary_key].eq(parent_table[options[:foreign_key] || reflection.primary_key_name])]
              end

              unless klass.descends_from_active_record?
                sti_column = aliased_table[klass.inheritance_column]
                sti_condition = sti_column.eq(klass.sti_name)
                klass.descendants.each {|subclass| sti_condition = sti_condition.or(sti_column.eq(subclass.sti_name)) }

                @join << sti_condition
              end

              [through_reflection, reflection].each do |ref|
                if ref && ref.options[:conditions]
                  @join << interpolate_sql(sanitize_sql(ref.options[:conditions], aliased_table_name))
                end
              end

              @join
            end

            def relation
              aliased = Arel::Table.new(table_name, :as => @aliased_table_name, :engine => arel_engine)

              if reflection.macro == :has_and_belongs_to_many
                [Arel::Table.new(options[:join_table], :as => aliased_join_table_name, :engine => arel_engine), aliased]
              elsif reflection.options[:through]
                [Arel::Table.new(through_reflection.klass.table_name, :as => aliased_join_table_name, :engine => arel_engine), aliased]
              else
                aliased
              end
            end

            def join_relation(joining_relation, join = nil)
              joining_relation.joins(self.with_join_class(Arel::OuterJoin))
            end

            protected

              def aliased_table_name_for(name, suffix = nil)
                if @join_dependency.table_aliases[name].zero?
                  @join_dependency.table_aliases[name] = @join_dependency.count_aliases_from_table_joins(name)
                end

                if !@join_dependency.table_aliases[name].zero? # We need an alias
                  name = active_record.connection.table_alias_for "#{pluralize(reflection.name)}_#{parent_table_name}#{suffix}"
                  @join_dependency.table_aliases[name] += 1
                  if @join_dependency.table_aliases[name] == 1 # First time we've seen this name
                    # Also need to count the aliases from the table_aliases to avoid incorrect count
                    @join_dependency.table_aliases[name] += @join_dependency.count_aliases_from_table_joins(name)
                  end
                  table_index = @join_dependency.table_aliases[name]
                  name = name[0..active_record.connection.table_alias_length-3] + "_#{table_index}" if table_index > 1
                else
                  @join_dependency.table_aliases[name] += 1
                end

                name
              end

              def pluralize(table_name)
                ActiveRecord::Base.pluralize_table_names ? table_name.to_s.pluralize : table_name
              end

              def table_alias_for(table_name, table_alias)
                 "#{table_name} #{table_alias if table_name != table_alias}".strip
              end

              def table_name_and_alias
                table_alias_for table_name, @aliased_table_name
              end

              def interpolate_sql(sql)
                instance_eval("%@#{sql.gsub('@', '\@')}@", __FILE__, __LINE__)
              end
          end
        end
    end
  end
end