aboutsummaryrefslogtreecommitdiffstats
path: root/activerecord/lib/active_record/migration.rb
blob: 0c50826c63068a2450d4b3744cd0323bb49403f2 (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
require "active_support/core_ext/module/attribute_accessors"
require 'set'

module ActiveRecord
  class MigrationError < ActiveRecordError#:nodoc:
    def initialize(message = nil)
      message = "\n\n#{message}\n\n" if message
      super
    end
  end

  # Exception that can be raised to stop migrations from being rolled back.
  # For example the following migration is not reversible.
  # Rolling back this migration will raise an ActiveRecord::IrreversibleMigration error.
  #
  #   class IrreversibleMigrationExample < ActiveRecord::Migration
  #     def change
  #       create_table :distributors do |t|
  #         t.string :zipcode
  #       end
  #
  #       execute <<-SQL
  #         ALTER TABLE distributors
  #           ADD CONSTRAINT zipchk
  #             CHECK (char_length(zipcode) = 5) NO INHERIT;
  #       SQL
  #     end
  #   end
  #
  # There are two ways to mitigate this problem.
  #
  # 1. Define <tt>#up</tt> and <tt>#down</tt> methods instead of <tt>#change</tt>:
  #
  #  class ReversibleMigrationExample < ActiveRecord::Migration
  #    def up
  #      create_table :distributors do |t|
  #        t.string :zipcode
  #      end
  #
  #      execute <<-SQL
  #        ALTER TABLE distributors
  #          ADD CONSTRAINT zipchk
  #            CHECK (char_length(zipcode) = 5) NO INHERIT;
  #      SQL
  #    end
  #
  #    def down
  #      execute <<-SQL
  #        ALTER TABLE distributors
  #          DROP CONSTRAINT zipchk
  #      SQL
  #
  #      drop_table :distributors
  #    end
  #  end
  #
  # 2. Use the #reversible method in <tt>#change</tt> method:
  #
  #   class ReversibleMigrationExample < ActiveRecord::Migration
  #     def change
  #       create_table :distributors do |t|
  #         t.string :zipcode
  #       end
  #
  #       reversible do |dir|
  #         dir.up do
  #           execute <<-SQL
  #             ALTER TABLE distributors
  #               ADD CONSTRAINT zipchk
  #                 CHECK (char_length(zipcode) = 5) NO INHERIT;
  #           SQL
  #         end
  #
  #         dir.down do
  #           execute <<-SQL
  #             ALTER TABLE distributors
  #               DROP CONSTRAINT zipchk
  #           SQL
  #         end
  #       end
  #     end
  #   end
  class IrreversibleMigration < MigrationError
  end

  class DuplicateMigrationVersionError < MigrationError#:nodoc:
    def initialize(version = nil)
      if version
        super("Multiple migrations have the version number #{version}.")
      else
        super("Duplicate migration version error.")
      end
    end
  end

  class DuplicateMigrationNameError < MigrationError#:nodoc:
    def initialize(name = nil)
      if name
        super("Multiple migrations have the name #{name}.")
      else
        super("Duplicate migration name.")
      end
    end
  end

  class UnknownMigrationVersionError < MigrationError #:nodoc:
    def initialize(version = nil)
      if version
        super("No migration with version number #{version}.")
      else
        super("Unknown migration version.")
      end
    end
  end

  class IllegalMigrationNameError < MigrationError#:nodoc:
    def initialize(name = nil)
      if name
        super("Illegal name for migration file: #{name}\n\t(only lower case letters, numbers, and '_' allowed).")
      else
        super("Illegal name for migration.")
      end
    end
  end

  class PendingMigrationError < MigrationError#:nodoc:
    def initialize(message = nil)
      if !message && defined?(Rails.env)
        super("Migrations are pending. To resolve this issue, run:\n\n\tbin/rake db:migrate RAILS_ENV=#{::Rails.env}.")
      elsif !message
        super("Migrations are pending. To resolve this issue, run:\n\n\tbin/rake db:migrate.")
      else
        super
      end
    end
  end

  # = Active Record Migrations
  #
  # Migrations can manage the evolution of a schema used by several physical
  # databases. It's a solution to the common problem of adding a field to make
  # a new feature work in your local database, but being unsure of how to
  # push that change to other developers and to the production server. With
  # migrations, you can describe the transformations in self-contained classes
  # that can be checked into version control systems and executed against
  # another database that might be one, two, or five versions behind.
  #
  # Example of a simple migration:
  #
  #   class AddSsl < ActiveRecord::Migration
  #     def up
  #       add_column :accounts, :ssl_enabled, :boolean, default: true
  #     end
  #
  #     def down
  #       remove_column :accounts, :ssl_enabled
  #     end
  #   end
  #
  # This migration will add a boolean flag to the accounts table and remove it
  # if you're backing out of the migration. It shows how all migrations have
  # two methods +up+ and +down+ that describes the transformations
  # required to implement or remove the migration. These methods can consist
  # of both the migration specific methods like +add_column+ and +remove_column+,
  # but may also contain regular Ruby code for generating data needed for the
  # transformations.
  #
  # Example of a more complex migration that also needs to initialize data:
  #
  #   class AddSystemSettings < ActiveRecord::Migration
  #     def up
  #       create_table :system_settings do |t|
  #         t.string  :name
  #         t.string  :label
  #         t.text    :value
  #         t.string  :type
  #         t.integer :position
  #       end
  #
  #       SystemSetting.create  name:  'notice',
  #                             label: 'Use notice?',
  #                             value: 1
  #     end
  #
  #     def down
  #       drop_table :system_settings
  #     end
  #   end
  #
  # This migration first adds the +system_settings+ table, then creates the very
  # first row in it using the Active Record model that relies on the table. It
  # also uses the more advanced +create_table+ syntax where you can specify a
  # complete table schema in one block call.
  #
  # == Available transformations
  #
  # * <tt>create_table(name, options)</tt>: Creates a table called +name+ and
  #   makes the table object available to a block that can then add columns to it,
  #   following the same format as +add_column+. See example above. The options hash
  #   is for fragments like "DEFAULT CHARSET=UTF-8" that are appended to the create
  #   table definition.
  # * <tt>drop_table(name)</tt>: Drops the table called +name+.
  # * <tt>change_table(name, options)</tt>: Allows to make column alterations to
  #   the table called +name+. It makes the table object available to a block that
  #   can then add/remove columns, indexes or foreign keys to it.
  # * <tt>rename_table(old_name, new_name)</tt>: Renames the table called +old_name+
  #   to +new_name+.
  # * <tt>add_column(table_name, column_name, type, options)</tt>: Adds a new column
  #   to the table called +table_name+
  #   named +column_name+ specified to be one of the following types:
  #   <tt>:string</tt>, <tt>:text</tt>, <tt>:integer</tt>, <tt>:float</tt>,
  #   <tt>:decimal</tt>, <tt>:datetime</tt>, <tt>:timestamp</tt>, <tt>:time</tt>,
  #   <tt>:date</tt>, <tt>:binary</tt>, <tt>:boolean</tt>. A default value can be
  #   specified by passing an +options+ hash like <tt>{ default: 11 }</tt>.
  #   Other options include <tt>:limit</tt> and <tt>:null</tt> (e.g.
  #   <tt>{ limit: 50, null: false }</tt>) -- see
  #   ActiveRecord::ConnectionAdapters::TableDefinition#column for details.
  # * <tt>rename_column(table_name, column_name, new_column_name)</tt>: Renames
  #   a column but keeps the type and content.
  # * <tt>change_column(table_name, column_name, type, options)</tt>:  Changes
  #   the column to a different type using the same parameters as add_column.
  # * <tt>remove_column(table_name, column_name, type, options)</tt>: Removes the column
  #   named +column_name+ from the table called +table_name+.
  # * <tt>add_index(table_name, column_names, options)</tt>: Adds a new index
  #   with the name of the column. Other options include
  #   <tt>:name</tt>, <tt>:unique</tt> (e.g.
  #   <tt>{ name: 'users_name_index', unique: true }</tt>) and <tt>:order</tt>
  #   (e.g. <tt>{ order: { name: :desc } }</tt>).
  # * <tt>remove_index(table_name, column: column_names)</tt>: Removes the index
  #   specified by +column_names+.
  # * <tt>remove_index(table_name, name: index_name)</tt>: Removes the index
  #   specified by +index_name+.
  # * <tt>add_reference(:table_name, :reference_name)</tt>: Adds a new column
  #   +reference_name_id+ by default an integer. See
  #   ActiveRecord::ConnectionAdapters::SchemaStatements#add_reference for details.
  #
  # == Irreversible transformations
  #
  # Some transformations are destructive in a manner that cannot be reversed.
  # Migrations of that kind should raise an <tt>ActiveRecord::IrreversibleMigration</tt>
  # exception in their +down+ method.
  #
  # == Running migrations from within Rails
  #
  # The Rails package has several tools to help create and apply migrations.
  #
  # To generate a new migration, you can use
  #   rails generate migration MyNewMigration
  #
  # where MyNewMigration is the name of your migration. The generator will
  # create an empty migration file <tt>timestamp_my_new_migration.rb</tt>
  # in the <tt>db/migrate/</tt> directory where <tt>timestamp</tt> is the
  # UTC formatted date and time that the migration was generated.
  #
  # There is a special syntactic shortcut to generate migrations that add fields to a table.
  #
  #   rails generate migration add_fieldname_to_tablename fieldname:string
  #
  # This will generate the file <tt>timestamp_add_fieldname_to_tablename.rb</tt>, which will look like this:
  #   class AddFieldnameToTablename < ActiveRecord::Migration
  #     def change
  #       add_column :tablenames, :fieldname, :string
  #     end
  #   end
  #
  # To run migrations against the currently configured database, use
  # <tt>rake db:migrate</tt>. This will update the database by running all of the
  # pending migrations, creating the <tt>schema_migrations</tt> table
  # (see "About the schema_migrations table" section below) if missing. It will also
  # invoke the db:schema:dump task, which will update your db/schema.rb file
  # to match the structure of your database.
  #
  # To roll the database back to a previous migration version, use
  # <tt>rake db:migrate VERSION=X</tt> where <tt>X</tt> is the version to which
  # you wish to downgrade. Alternatively, you can also use the STEP option if you
  # wish to rollback last few migrations. <tt>rake db:migrate STEP=2</tt> will rollback
  # the latest two migrations.
  #
  # If any of the migrations throw an <tt>ActiveRecord::IrreversibleMigration</tt> exception,
  # that step will fail and you'll have some manual work to do.
  #
  # == Database support
  #
  # Migrations are currently supported in MySQL, PostgreSQL, SQLite,
  # SQL Server, and Oracle (all supported databases except DB2).
  #
  # == More examples
  #
  # Not all migrations change the schema. Some just fix the data:
  #
  #   class RemoveEmptyTags < ActiveRecord::Migration
  #     def up
  #       Tag.all.each { |tag| tag.destroy if tag.pages.empty? }
  #     end
  #
  #     def down
  #       # not much we can do to restore deleted data
  #       raise ActiveRecord::IrreversibleMigration, "Can't recover the deleted tags"
  #     end
  #   end
  #
  # Others remove columns when they migrate up instead of down:
  #
  #   class RemoveUnnecessaryItemAttributes < ActiveRecord::Migration
  #     def up
  #       remove_column :items, :incomplete_items_count
  #       remove_column :items, :completed_items_count
  #     end
  #
  #     def down
  #       add_column :items, :incomplete_items_count
  #       add_column :items, :completed_items_count
  #     end
  #   end
  #
  # And sometimes you need to do something in SQL not abstracted directly by migrations:
  #
  #   class MakeJoinUnique < ActiveRecord::Migration
  #     def up
  #       execute "ALTER TABLE `pages_linked_pages` ADD UNIQUE `page_id_linked_page_id` (`page_id`,`linked_page_id`)"
  #     end
  #
  #     def down
  #       execute "ALTER TABLE `pages_linked_pages` DROP INDEX `page_id_linked_page_id`"
  #     end
  #   end
  #
  # == Using a model after changing its table
  #
  # Sometimes you'll want to add a column in a migration and populate it
  # immediately after. In that case, you'll need to make a call to
  # <tt>Base#reset_column_information</tt> in order to ensure that the model has the
  # latest column data from after the new column was added. Example:
  #
  #   class AddPeopleSalary < ActiveRecord::Migration
  #     def up
  #       add_column :people, :salary, :integer
  #       Person.reset_column_information
  #       Person.all.each do |p|
  #         p.update_attribute :salary, SalaryCalculator.compute(p)
  #       end
  #     end
  #   end
  #
  # == Controlling verbosity
  #
  # By default, migrations will describe the actions they are taking, writing
  # them to the console as they happen, along with benchmarks describing how
  # long each step took.
  #
  # You can quiet them down by setting ActiveRecord::Migration.verbose = false.
  #
  # You can also insert your own messages and benchmarks by using the +say_with_time+
  # method:
  #
  #   def up
  #     ...
  #     say_with_time "Updating salaries..." do
  #       Person.all.each do |p|
  #         p.update_attribute :salary, SalaryCalculator.compute(p)
  #       end
  #     end
  #     ...
  #   end
  #
  # The phrase "Updating salaries..." would then be printed, along with the
  # benchmark for the block when the block completes.
  #
  # == Timestamped Migrations
  #
  # By default, Rails generates migrations that look like:
  #
  #    20080717013526_your_migration_name.rb
  #
  # The prefix is a generation timestamp (in UTC).
  #
  # If you'd prefer to use numeric prefixes, you can turn timestamped migrations
  # off by setting:
  #
  #    config.active_record.timestamped_migrations = false
  #
  # In application.rb.
  #
  # == Reversible Migrations
  #
  # Reversible migrations are migrations that know how to go +down+ for you.
  # You simply supply the +up+ logic, and the Migration system figures out
  # how to execute the down commands for you.
  #
  # To define a reversible migration, define the +change+ method in your
  # migration like this:
  #
  #   class TenderloveMigration < ActiveRecord::Migration
  #     def change
  #       create_table(:horses) do |t|
  #         t.column :content, :text
  #         t.column :remind_at, :datetime
  #       end
  #     end
  #   end
  #
  # This migration will create the horses table for you on the way up, and
  # automatically figure out how to drop the table on the way down.
  #
  # Some commands like +remove_column+ cannot be reversed.  If you care to
  # define how to move up and down in these cases, you should define the +up+
  # and +down+ methods as before.
  #
  # If a command cannot be reversed, an
  # <tt>ActiveRecord::IrreversibleMigration</tt> exception will be raised when
  # the migration is moving down.
  #
  # For a list of commands that are reversible, please see
  # <tt>ActiveRecord::Migration::CommandRecorder</tt>.
  #
  # == Transactional Migrations
  #
  # If the database adapter supports DDL transactions, all migrations will
  # automatically be wrapped in a transaction. There are queries that you
  # can't execute inside a transaction though, and for these situations
  # you can turn the automatic transactions off.
  #
  #   class ChangeEnum < ActiveRecord::Migration
  #     disable_ddl_transaction!
  #
  #     def up
  #       execute "ALTER TYPE model_size ADD VALUE 'new_value'"
  #     end
  #   end
  #
  # Remember that you can still open your own transactions, even if you
  # are in a Migration with <tt>self.disable_ddl_transaction!</tt>.
  class Migration
    autoload :CommandRecorder, 'active_record/migration/command_recorder'


    # This class is used to verify that all migrations have been run before
    # loading a web page if config.active_record.migration_error is set to :page_load
    class CheckPending
      def initialize(app)
        @app = app
        @last_check = 0
      end

      def call(env)
        if connection.supports_migrations?
          mtime = ActiveRecord::Migrator.last_migration.mtime.to_i
          if @last_check < mtime
            ActiveRecord::Migration.check_pending!(connection)
            @last_check = mtime
          end
        end
        @app.call(env)
      end

      private

      def connection
        ActiveRecord::Base.connection
      end
    end

    class << self
      attr_accessor :delegate # :nodoc:
      attr_accessor :disable_ddl_transaction # :nodoc:

      # Raises <tt>ActiveRecord::PendingMigrationError</tt> error if any migrations are pending.
      def check_pending!(connection = Base.connection)
        raise ActiveRecord::PendingMigrationError if ActiveRecord::Migrator.needs_migration?(connection)
      end

      def load_schema_if_pending!
        if ActiveRecord::Migrator.needs_migration? || !ActiveRecord::Migrator.any_migrations?
          # Roundtrip to Rake to allow plugins to hook into database initialization.
          FileUtils.cd Rails.root do
            current_config = Base.connection_config
            Base.clear_all_connections!
            system("bin/rake db:test:prepare")
            # Establish a new connection, the old database may be gone (db:test:prepare uses purge)
            Base.establish_connection(current_config)
          end
          check_pending!
        end
      end

      def maintain_test_schema! # :nodoc:
        if ActiveRecord::Base.maintain_test_schema
          suppress_messages { load_schema_if_pending! }
        end
      end

      def method_missing(name, *args, &block) # :nodoc:
        (delegate || superclass.delegate).send(name, *args, &block)
      end

      def migrate(direction)
        new.migrate direction
      end

      # Disable the transaction wrapping this migration.
      # You can still create your own transactions even after calling #disable_ddl_transaction!
      #
      # For more details read the {"Transactional Migrations" section above}[rdoc-ref:Migration].
      def disable_ddl_transaction!
        @disable_ddl_transaction = true
      end
    end

    def disable_ddl_transaction # :nodoc:
      self.class.disable_ddl_transaction
    end

    cattr_accessor :verbose
    attr_accessor :name, :version

    def initialize(name = self.class.name, version = nil)
      @name       = name
      @version    = version
      @connection = nil
    end

    self.verbose = true
    # instantiate the delegate object after initialize is defined
    self.delegate = new

    # Reverses the migration commands for the given block and
    # the given migrations.
    #
    # The following migration will remove the table 'horses'
    # and create the table 'apples' on the way up, and the reverse
    # on the way down.
    #
    #   class FixTLMigration < ActiveRecord::Migration
    #     def change
    #       revert do
    #         create_table(:horses) do |t|
    #           t.text :content
    #           t.datetime :remind_at
    #         end
    #       end
    #       create_table(:apples) do |t|
    #         t.string :variety
    #       end
    #     end
    #   end
    #
    # Or equivalently, if +TenderloveMigration+ is defined as in the
    # documentation for Migration:
    #
    #   require_relative '20121212123456_tenderlove_migration'
    #
    #   class FixupTLMigration < ActiveRecord::Migration
    #     def change
    #       revert TenderloveMigration
    #
    #       create_table(:apples) do |t|
    #         t.string :variety
    #       end
    #     end
    #   end
    #
    # This command can be nested.
    def revert(*migration_classes)
      run(*migration_classes.reverse, revert: true) unless migration_classes.empty?
      if block_given?
        if connection.respond_to? :revert
          connection.revert { yield }
        else
          recorder = CommandRecorder.new(connection)
          @connection = recorder
          suppress_messages do
            connection.revert { yield }
          end
          @connection = recorder.delegate
          recorder.commands.each do |cmd, args, block|
            send(cmd, *args, &block)
          end
        end
      end
    end

    def reverting?
      connection.respond_to?(:reverting) && connection.reverting
    end

    class ReversibleBlockHelper < Struct.new(:reverting) # :nodoc:
      def up
        yield unless reverting
      end

      def down
        yield if reverting
      end
    end

    # Used to specify an operation that can be run in one direction or another.
    # Call the methods +up+ and +down+ of the yielded object to run a block
    # only in one given direction.
    # The whole block will be called in the right order within the migration.
    #
    # In the following example, the looping on users will always be done
    # when the three columns 'first_name', 'last_name' and 'full_name' exist,
    # even when migrating down:
    #
    #    class SplitNameMigration < ActiveRecord::Migration
    #      def change
    #        add_column :users, :first_name, :string
    #        add_column :users, :last_name, :string
    #
    #        reversible do |dir|
    #          User.reset_column_information
    #          User.all.each do |u|
    #            dir.up   { u.first_name, u.last_name = u.full_name.split(' ') }
    #            dir.down { u.full_name = "#{u.first_name} #{u.last_name}" }
    #            u.save
    #          end
    #        end
    #
    #        revert { add_column :users, :full_name, :string }
    #      end
    #    end
    def reversible
      helper = ReversibleBlockHelper.new(reverting?)
      execute_block{ yield helper }
    end

    # Runs the given migration classes.
    # Last argument can specify options:
    # - :direction (default is :up)
    # - :revert (default is false)
    def run(*migration_classes)
      opts = migration_classes.extract_options!
      dir = opts[:direction] || :up
      dir = (dir == :down ? :up : :down) if opts[:revert]
      if reverting?
        # If in revert and going :up, say, we want to execute :down without reverting, so
        revert { run(*migration_classes, direction: dir, revert: true) }
      else
        migration_classes.each do |migration_class|
          migration_class.new.exec_migration(connection, dir)
        end
      end
    end

    def up
      self.class.delegate = self
      return unless self.class.respond_to?(:up)
      self.class.up
    end

    def down
      self.class.delegate = self
      return unless self.class.respond_to?(:down)
      self.class.down
    end

    # Execute this migration in the named direction
    def migrate(direction)
      return unless respond_to?(direction)

      case direction
      when :up   then announce "migrating"
      when :down then announce "reverting"
      end

      time   = nil
      ActiveRecord::Base.connection_pool.with_connection do |conn|
        time = Benchmark.measure do
          exec_migration(conn, direction)
        end
      end

      case direction
      when :up   then announce "migrated (%.4fs)" % time.real; write
      when :down then announce "reverted (%.4fs)" % time.real; write
      end
    end

    def exec_migration(conn, direction)
      @connection = conn
      if respond_to?(:change)
        if direction == :down
          revert { change }
        else
          change
        end
      else
        send(direction)
      end
    ensure
      @connection = nil
    end

    def write(text="")
      puts(text) if verbose
    end

    def announce(message)
      text = "#{version} #{name}: #{message}"
      length = [0, 75 - text.length].max
      write "== %s %s" % [text, "=" * length]
    end

    def say(message, subitem=false)
      write "#{subitem ? "   ->" : "--"} #{message}"
    end

    def say_with_time(message)
      say(message)
      result = nil
      time = Benchmark.measure { result = yield }
      say "%.4fs" % time.real, :subitem
      say("#{result} rows", :subitem) if result.is_a?(Integer)
      result
    end

    def suppress_messages
      save, self.verbose = verbose, false
      yield
    ensure
      self.verbose = save
    end

    def connection
      @connection || ActiveRecord::Base.connection
    end

    def method_missing(method, *arguments, &block)
      arg_list = arguments.map(&:inspect) * ', '

      say_with_time "#{method}(#{arg_list})" do
        unless connection.respond_to? :revert
          unless arguments.empty? || [:execute, :enable_extension, :disable_extension].include?(method)
            arguments[0] = proper_table_name(arguments.first, table_name_options)
            if [:rename_table, :add_foreign_key].include?(method) ||
              (method == :remove_foreign_key && !arguments.second.is_a?(Hash))
              arguments[1] = proper_table_name(arguments.second, table_name_options)
            end
          end
        end
        return super unless connection.respond_to?(method)
        connection.send(method, *arguments, &block)
      end
    end

    def copy(destination, sources, options = {})
      copied = []

      FileUtils.mkdir_p(destination) unless File.exist?(destination)

      destination_migrations = ActiveRecord::Migrator.migrations(destination)
      last = destination_migrations.last
      sources.each do |scope, path|
        source_migrations = ActiveRecord::Migrator.migrations(path)

        source_migrations.each do |migration|
          source = File.binread(migration.filename)
          inserted_comment = "# This migration comes from #{scope} (originally #{migration.version})\n"
          if /\A#.*\b(?:en)?coding:\s*\S+/ =~ source
            # If we have a magic comment in the original migration,
            # insert our comment after the first newline(end of the magic comment line)
            # so the magic keep working.
            # Note that magic comments must be at the first line(except sh-bang).
            source[/\n/] = "\n#{inserted_comment}"
          else
            source = "#{inserted_comment}#{source}"
          end

          if duplicate = destination_migrations.detect { |m| m.name == migration.name }
            if options[:on_skip] && duplicate.scope != scope.to_s
              options[:on_skip].call(scope, migration)
            end
            next
          end

          migration.version = next_migration_number(last ? last.version + 1 : 0).to_i
          new_path = File.join(destination, "#{migration.version}_#{migration.name.underscore}.#{scope}.rb")
          old_path, migration.filename = migration.filename, new_path
          last = migration

          File.binwrite(migration.filename, source)
          copied << migration
          options[:on_copy].call(scope, migration, old_path) if options[:on_copy]
          destination_migrations << migration
        end
      end

      copied
    end

    # Finds the correct table name given an Active Record object.
    # Uses the Active Record object's own table_name, or pre/suffix from the
    # options passed in.
    def proper_table_name(name, options = {})
      if name.respond_to? :table_name
        name.table_name
      else
        "#{options[:table_name_prefix]}#{name}#{options[:table_name_suffix]}"
      end
    end

    # Determines the version number of the next migration.
    def next_migration_number(number)
      if ActiveRecord::Base.timestamped_migrations
        [Time.now.utc.strftime("%Y%m%d%H%M%S"), "%.14d" % number].max
      else
        SchemaMigration.normalize_migration_number(number)
      end
    end

    # Builds a hash for use in ActiveRecord::Migration#proper_table_name using
    # the Active Record object's table_name prefix and suffix
    def table_name_options(config = ActiveRecord::Base) #:nodoc:
      {
        table_name_prefix: config.table_name_prefix,
        table_name_suffix: config.table_name_suffix
      }
    end

    private
    def execute_block
      if connection.respond_to? :execute_block
        super # use normal delegation to record the block
      else
        yield
      end
    end
  end

  # MigrationProxy is used to defer loading of the actual migration classes
  # until they are needed
  class MigrationProxy < Struct.new(:name, :version, :filename, :scope)

    def initialize(name, version, filename, scope)
      super
      @migration = nil
    end

    def basename
      File.basename(filename)
    end

    def mtime
      File.mtime filename
    end

    delegate :migrate, :announce, :write, :disable_ddl_transaction, to: :migration

    private

      def migration
        @migration ||= load_migration
      end

      def load_migration
        require(File.expand_path(filename))
        name.constantize.new(name, version)
      end

  end

  class NullMigration < MigrationProxy #:nodoc:
    def initialize
      super(nil, 0, nil, nil)
    end

    def mtime
      0
    end
  end

  class Migrator#:nodoc:
    class << self
      attr_writer :migrations_paths
      alias :migrations_path= :migrations_paths=

      def migrate(migrations_paths, target_version = nil, &block)
        case
        when target_version.nil?
          up(migrations_paths, target_version, &block)
        when current_version == 0 && target_version == 0
          []
        when current_version > target_version
          down(migrations_paths, target_version, &block)
        else
          up(migrations_paths, target_version, &block)
        end
      end

      def rollback(migrations_paths, steps=1)
        move(:down, migrations_paths, steps)
      end

      def forward(migrations_paths, steps=1)
        move(:up, migrations_paths, steps)
      end

      def up(migrations_paths, target_version = nil)
        migrations = migrations(migrations_paths)
        migrations.select! { |m| yield m } if block_given?

        new(:up, migrations, target_version).migrate
      end

      def down(migrations_paths, target_version = nil)
        migrations = migrations(migrations_paths)
        migrations.select! { |m| yield m } if block_given?

        new(:down, migrations, target_version).migrate
      end

      def run(direction, migrations_paths, target_version)
        new(direction, migrations(migrations_paths), target_version).run
      end

      def open(migrations_paths)
        new(:up, migrations(migrations_paths), nil)
      end

      def schema_migrations_table_name
        SchemaMigration.table_name
      end

      def get_all_versions(connection = Base.connection)
        if connection.table_exists?(schema_migrations_table_name)
          SchemaMigration.all.map { |x| x.version.to_i }.sort
        else
          []
        end
      end

      def current_version(connection = Base.connection)
        get_all_versions(connection).max || 0
      end

      def needs_migration?(connection = Base.connection)
        (migrations(migrations_paths).collect(&:version) - get_all_versions(connection)).size > 0
      end

      def any_migrations?
        migrations(migrations_paths).any?
      end

      def last_migration #:nodoc:
        migrations(migrations_paths).last || NullMigration.new
      end

      def migrations_paths
        @migrations_paths ||= ['db/migrate']
        # just to not break things if someone uses: migration_path = some_string
        Array(@migrations_paths)
      end

      def migrations(paths)
        paths = Array(paths)

        files = Dir[*paths.map { |p| "#{p}/**/[0-9]*_*.rb" }]

        migrations = files.map do |file|
          version, name, scope = file.scan(/([0-9]+)_([_a-z0-9]*)\.?([_a-z0-9]*)?\.rb\z/).first

          raise IllegalMigrationNameError.new(file) unless version
          version = version.to_i
          name = name.camelize

          MigrationProxy.new(name, version, file, scope)
        end

        migrations.sort_by(&:version)
      end

      private

      def move(direction, migrations_paths, steps)
        migrator = new(direction, migrations(migrations_paths))
        start_index = migrator.migrations.index(migrator.current_migration)

        if start_index
          finish = migrator.migrations[start_index + steps]
          version = finish ? finish.version : 0
          send(direction, migrations_paths, version)
        end
      end
    end

    def initialize(direction, migrations, target_version = nil)
      raise StandardError.new("This database does not yet support migrations") unless Base.connection.supports_migrations?

      @direction         = direction
      @target_version    = target_version
      @migrated_versions = nil
      @migrations        = migrations

      validate(@migrations)

      Base.connection.initialize_schema_migrations_table
    end

    def current_version
      migrated.max || 0
    end

    def current_migration
      migrations.detect { |m| m.version == current_version }
    end
    alias :current :current_migration

    def run
      migration = migrations.detect { |m| m.version == @target_version }
      raise UnknownMigrationVersionError.new(@target_version) if migration.nil?
      unless (up? && migrated.include?(migration.version.to_i)) || (down? && !migrated.include?(migration.version.to_i))
        begin
          execute_migration_in_transaction(migration, @direction)
        rescue => e
          canceled_msg = use_transaction?(migration) ? ", this migration was canceled" : ""
          raise StandardError, "An error has occurred#{canceled_msg}:\n\n#{e}", e.backtrace
        end
      end
    end

    def migrate
      if !target && @target_version && @target_version > 0
        raise UnknownMigrationVersionError.new(@target_version)
      end

      runnable.each do |migration|
        Base.logger.info "Migrating to #{migration.name} (#{migration.version})" if Base.logger

        begin
          execute_migration_in_transaction(migration, @direction)
        rescue => e
          canceled_msg = use_transaction?(migration) ? "this and " : ""
          raise StandardError, "An error has occurred, #{canceled_msg}all later migrations canceled:\n\n#{e}", e.backtrace
        end
      end
    end

    def runnable
      runnable = migrations[start..finish]
      if up?
        runnable.reject { |m| ran?(m) }
      else
        # skip the last migration if we're headed down, but not ALL the way down
        runnable.pop if target
        runnable.find_all { |m| ran?(m) }
      end
    end

    def migrations
      down? ? @migrations.reverse : @migrations.sort_by(&:version)
    end

    def pending_migrations
      already_migrated = migrated
      migrations.reject { |m| already_migrated.include?(m.version) }
    end

    def migrated
      @migrated_versions ||= Set.new(self.class.get_all_versions)
    end

    private
    def ran?(migration)
      migrated.include?(migration.version.to_i)
    end

    def execute_migration_in_transaction(migration, direction)
      ddl_transaction(migration) do
        migration.migrate(direction)
        record_version_state_after_migrating(migration.version)
      end
    end

    def target
      migrations.detect { |m| m.version == @target_version }
    end

    def finish
      migrations.index(target) || migrations.size - 1
    end

    def start
      up? ? 0 : (migrations.index(current) || 0)
    end

    def validate(migrations)
      name ,= migrations.group_by(&:name).find { |_,v| v.length > 1 }
      raise DuplicateMigrationNameError.new(name) if name

      version ,= migrations.group_by(&:version).find { |_,v| v.length > 1 }
      raise DuplicateMigrationVersionError.new(version) if version
    end

    def record_version_state_after_migrating(version)
      if down?
        migrated.delete(version)
        ActiveRecord::SchemaMigration.where(:version => version.to_s).delete_all
      else
        migrated << version
        ActiveRecord::SchemaMigration.create!(:version => version.to_s)
      end
    end

    def up?
      @direction == :up
    end

    def down?
      @direction == :down
    end

    # Wrap the migration in a transaction only if supported by the adapter.
    def ddl_transaction(migration)
      if use_transaction?(migration)
        Base.transaction { yield }
      else
        yield
      end
    end

    def use_transaction?(migration)
      !migration.disable_ddl_transaction && Base.connection.supports_ddl_transactions?
    end
  end
end