aboutsummaryrefslogtreecommitdiffstats
path: root/railties/guides/source/layouts_and_rendering.textile
blob: 4e26d152bf7b470d9b02bfce794218d98e7b7bd4 (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
h2. Layouts and Rendering in Rails

This guide covers the basic layout features of Action Controller and Action View. By referring to this guide, you will be able to:

* Use the various rendering methods built into Rails
* Create layouts with multiple content sections
* Use partials to DRY up your views
* Use nested layouts (sub-templates)

endprologue.

h3. Overview: How the Pieces Fit Together

This guide focuses on the interaction between Controller and View in the Model-View-Controller triangle. As you know, the Controller is responsible for orchestrating the whole process of handling a request in Rails, though it normally hands off any heavy code to the Model. But then, when it's time to send a response back to the user, the Controller hands things off to the View. It's that handoff that is the subject of this guide.

In broad strokes, this involves deciding what should be sent as the response and calling an appropriate method to create that response. If the response is a full-blown view, Rails also does some extra work to wrap the view in a layout and possibly to pull in partial views. You'll see all of those paths later in this guide.

h3. Creating Responses

From the controller's point of view, there are three ways to create an HTTP response:

* Call +render+ to create a full response to send back to the browser
* Call +redirect_to+ to send an HTTP redirect status code to the browser
* Call +head+ to create a response consisting solely of HTTP headers to send back to the browser

I'll cover each of these methods in turn. But first, a few words about the very easiest thing that the controller can do to create a response: nothing at all.

h4. Rendering by Default: Convention Over Configuration in Action

You've heard that Rails promotes "convention over configuration". Default rendering is an excellent example of this. By default, controllers in Rails automatically render views with names that correspond to valid routes. For example, if you have this code in your +BooksController+ class:

<ruby>
class BooksController < ApplicationController
end
</ruby>

And the following in your routes file:

<ruby>
resources :books
</ruby>

And you have a view file +app/views/books/index.html.erb+:

<ruby>
<h1>Books are coming soon!</h1>
</ruby>

Rails will automatically render +app/views/books/index.html.erb+ when you navigate to +/books+ and you will see "Books are coming soon!" on your screen.

However a coming soon screen is only minimally useful, so you will soon create your +Book+ model and add the index action to +BooksController+:

<ruby>
class BooksController < ApplicationController
  def index
    @books = Book.all
  end
end
</ruby>

Note that we don't have explicit render at the end of the index action in accordance with "convention over configuration" principle. The rule is that if you do not explicitly render something at the end of a controller action, Rails will automatically look for the +action_name.html.erb+ template in the controller's view path and render it. So in this case, Rails will render the +app/views/books/index.html.erb+ file.

If we want to display the properties of all the books in our view, we can do so with an ERB template like this:

<ruby>
<h1>Listing Books</h1>

<table>
  <tr>
    <th>Title</th>
    <th>Summary</th>
    <th></th>
    <th></th>
    <th></th>
  </tr>

<% @books.each do |book| %>
  <tr>
    <td><%= book.title %></td>
    <td><%= book.content %></td>
    <td><%= link_to 'Show', book %></td>
    <td><%= link_to 'Edit', edit_book_path(book) %></td>
    <td><%= link_to 'Remove', book, :confirm => 'Are you sure?', :method => :delete %></td>
  </tr>
<% end %>
</table>

<br />

<%= link_to 'New book', new_book_path %>
</ruby>

NOTE: The actual rendering is done by subclasses of +ActionView::TemplateHandlers+. This guide does not dig into that process, but it's important to know that the file extension on your view controls the choice of template handler. In Rails 2, the standard extensions are +.erb+ for ERB (HTML with embedded Ruby), +.rjs+ for RJS (JavaScript with embedded ruby) and +.builder+ for Builder (XML generator).

h4. Using +render+

In most cases, the +ActionController::Base#render+ method does the heavy lifting of rendering your application's content for use by a browser. There are a variety of ways to customise the behaviour of +render+. You can render the default view for a Rails template, or a specific template, or a file, or inline code, or nothing at all. You can render text, JSON, or XML. You can specify the content type or HTTP status of the rendered response as well.

TIP: If you want to see the exact results of a call to +render+ without needing to inspect it in a browser, you can call +render_to_string+. This method takes exactly the same options as +render+, but it returns a string instead of sending a response back to the browser.

h5. Rendering Nothing

Perhaps the simplest thing you can do with +render+ is to render nothing at all:

<ruby>
render :nothing => true
</ruby>

If you look at the response for this using Curl you will see the following:

<shell>
 $ curl -i 127.0.0.1:3000/books
HTTP/1.1 200 OK
Connection: close
Date: Sun, 24 Jan 2010 09:25:18 GMT
Transfer-Encoding: chunked
Content-Type: */*; charset=utf-8
X-Runtime: 0.014297
Set-Cookie: _blog_session=...snip...; path=/; HttpOnly
Cache-Control: no-cache


 $
</shell>

We see there is an empty response (no data after the +Cache-Control+ line), but the request was successful because Rails has set the response to 200 OK.  You can set the +:status+ options on render to change this response. Rendering nothing can be useful for AJAX requests where all you want to send back to the browser is an acknowledgement that the request was completed.

TIP: You should probably be using the +head+ method, discussed later in this guide, instead of +render :nothing+. This provides additional flexibility and makes it explicit that you're only generating HTTP headers.

h5. Rendering an Action's View

If you want to render the view that corresponds to a different action within the same template, you can use +render+ with the name of the view:

<ruby>
def update
  @book = Book.find(params[:id])
  if @book.update_attributes(params[:book])
    redirect_to(@book)
  else
    render "edit"
  end
end
</ruby>

If the call to +update_attributes+ fails, calling the +update+ action in this controller will render the +edit.html.erb+ template belonging to the same controller.

If you prefer, you can use a symbol instead of a string to specify the action to render:

<ruby>
def update
  @book = Book.find(params[:id])
  if @book.update_attributes(params[:book])
    redirect_to(@book)
  else
    render :edit
  end
end
</ruby>

To be explicit, you can use +render+ with the +:action+ option (though this is no longer necessary in Rails 3.0):

<ruby>
def update
  @book = Book.find(params[:id])
  if @book.update_attributes(params[:book])
    redirect_to(@book)
  else
    render :action => "edit"
  end
end
</ruby>

WARNING: Using +render+ with +:action+ is a frequent source of confusion for Rails newcomers. The specified action is used to determine which view to render, but Rails does _not_ run any of the code for that action in the controller. Any instance variables that you require in the view must be set up in the current action before calling +render+.

h5. Rendering an Action's Template from Another Controller

What if you want to render a template from an entirely different controller from the one that contains the action code? You can also do that with +render+, which accepts the full path (relative to +app/views+) of the template to render. For example, if you're running code in an +AdminProductsController+ that lives in +app/controllers/admin+, you can render the results of an action to a template in +app/views/products+ this way:

<ruby>
render 'products/show'
</ruby>

Rails knows that this view belongs to a different controller because of the embedded slash character in the string. If you want to be explicit, you can use the +:template+ option (which was required on Rails 2.2 and earlier):

<ruby>
render :template => 'products/show'
</ruby>

h5. Rendering an Arbitrary File

The +render+ method can also use a view that's entirely outside of your application (perhaps you're sharing views between two Rails applications):

<ruby>
render "/u/apps/warehouse_app/current/app/views/products/show"
</ruby>

Rails determines that this is a file render because of the leading slash character. To be explicit, you can use the +:file+ option (which was required on Rails 2.2 and earlier):

<ruby>
render :file =>
  "/u/apps/warehouse_app/current/app/views/products/show"
</ruby>

The +:file+ option takes an absolute file-system path. Of course, you need to have rights to the view that you're using to render the content.

NOTE: By default, the file is rendered without using the current layout. If you want Rails to put the file into the current layout, you need to add the +:layout => true+ option.

TIP: If you're running Rails on Microsoft Windows, you should use the +:file+ option to render a file, because Windows filenames do not have the same format as Unix filenames.

h5. Wrapping it up

The above three ways of rendering (rendering another template within the controller, rendering a template within another controller and rendering an arbitrary file on the file system) are actually variants of the same action.

In fact, in the BooksController class, inside of the edit action where we want to render the edit template if the book does not update successfully, all of the following render calls would all render the +edit.html.erb+ template in the +views/books+ directory:

<ruby>
render :edit
render :action => :edit
render 'edit'
render 'edit.html.erb'
render :action => 'edit'
render :action => 'edit.html.erb'
render 'books/edit'
render 'books/edit.html.erb'
render :template => 'books/edit'
render :template => 'books/edit.html.erb'
render '/path/to/rails/app/views/books/edit'
render '/path/to/rails/app/views/books/edit.html.erb'
render :file => '/path/to/rails/app/views/books/edit'
render :file => '/path/to/rails/app/views/books/edit.html.erb'
</ruby>

Which one you use is really a matter of style and convention, but the rule of thumb is to use the simplest one that makes sense for the code you are writing.

h5. Using +render+ with +:inline+

The +render+ method can do without a view completely, if you're willing to use the +:inline+ option to supply ERB as part of the method call. This is perfectly valid:

<ruby>
render :inline =>
  "<% products.each do |p| %><p><%= p.name %></p><% end %>"
</ruby>

WARNING: There is seldom any good reason to use this option. Mixing ERB into your controllers defeats the MVC orientation of Rails and will make it harder for other developers to follow the logic of your project. Use a separate erb view instead.

By default, inline rendering uses ERB. You can force it to use Builder instead with the +:type+ option:

<ruby>
render :inline =>
  "xml.p {'Horrid coding practice!'}", :type => :builder
</ruby>

h5. Using +render+ with +:update+

You can also render JavaScript-based page updates inline using the +:update+ option to +render+:

<ruby>
render :update do |page|
  page.replace_html 'warning', "Invalid options supplied"
end
</ruby>

WARNING: Placing JavaScript updates in your controller may seem to streamline small updates, but it defeats the MVC orientation of Rails and will make it harder for other developers to follow the logic of your project. We recommend using a separate RJS template instead, no matter how small the update.

h5. Rendering Text

You can send plain text - with no markup at all - back to the browser by using the +:text+ option to +render+:

<ruby>
render :text => "OK"
</ruby>

TIP: Rendering pure text is most useful when you're responding to AJAX or web service requests that are expecting something other than proper HTML.

NOTE: By default, if you use the +:text+ option, the text is rendered without using the current layout. If you want Rails to put the text into the current layout, you need to add the +:layout => true+ option.

h5. Rendering JSON

JSON is a JavaScript data format used by many AJAX libraries. Rails has built-in support for converting objects to JSON and rendering that JSON back to the browser:

<ruby>
render :json => @product
</ruby>

TIP: You don't need to call +to_json+ on the object that you want to render. If you use the +:json+ option, +render+ will automatically call +to_json+ for you.

h5. Rendering XML

Rails also has built-in support for converting objects to XML and rendering that XML back to the caller:

<ruby>
render :xml => @product
</ruby>

TIP: You don't need to call +to_xml+ on the object that you want to render. If you use the +:xml+ option, +render+ will automatically call +to_xml+ for you.

h5. Rendering Vanilla JavaScript

Rails can render vanilla JavaScript (as an alternative to using +update+ with an +.rjs+ file):

<ruby>
render :js => "alert('Hello Rails');"
</ruby>

This will send the supplied string to the browser with a MIME type of +text/javascript+.

h5. Options for +render+

Calls to the +render+ method generally accept four options:

* +:content_type+
* +:layout+
* +:status+
* +:location+

h6. The +:content_type+ Option

By default, Rails will serve the results of a rendering operation with the MIME content-type of +text/html+ (or +application/json+ if you use the +:json+ option, or +application/xml+ for the +:xml+ option.). There are times when you might like to change this, and you can do so by setting the +:content_type+ option:

<ruby>
render :file => filename, :content_type => 'application/rss'
</ruby>

h6. The +:layout+ Option

With most of the options to +render+, the rendered content is displayed as part of the current layout. You'll learn more about layouts and how to use them later in this guide.

You can use the +:layout+ option to tell Rails to use a specific file as the layout for the current action:

<ruby>
render :layout => 'special_layout'
</ruby>

You can also tell Rails to render with no layout at all:

<ruby>
render :layout => false
</ruby>

h6. The +:status+ Option

Rails will automatically generate a response with the correct HTML status code (in most cases, this is +200 OK+). You can use the +:status+ option to change this:

<ruby>
render :status => 500
render :status => :forbidden
</ruby>

Rails understands either numeric status codes or symbols for status codes. You can find its list of status codes in +actionpack/lib/action_controller/status_codes.rb+. You can also see there how Rails maps symbols to status codes.

h6. The +:location+ Option

You can use the +:location+ option to set the HTTP +Location+ header:

<ruby>
render :xml => photo, :location => photo_url(photo)
</ruby>

h5. Finding Layouts

To find the current layout, Rails first looks for a file in +app/views/layouts+ with the same base name as the controller. For example, rendering actions from the +PhotosController+ class will use +app/views/layouts/photos.html.erb+ (or +app/views/layouts/photos.builder+). If there is no such controller-specific layout, Rails will use +app/views/layouts/application.html.erb+ or +app/views/layouts/application.builder+. If there is no +.erb+ layout, Rails will use a +.builder+ layout if one exists. Rails also provides several ways to more precisely assign specific layouts to individual controllers and actions.

h6. Specifying Layouts on a per-Controller Basis

You can override the automatic layout conventions in your controllers by using the +layout+ declaration in the controller. For example:

<ruby>
class ProductsController < ApplicationController
  layout "inventory"
  #...
end
</ruby>

With this declaration, all methods within +ProductsController+ will use +app/views/layouts/inventory.html.erb+ for their layout.

To assign a specific layout for the entire application, use a declaration in your +ApplicationController+ class:

<ruby>
class ApplicationController < ActionController::Base
  layout "main"
  #...
end
</ruby>

With this declaration, all views in the entire application will use +app/views/layouts/main.html.erb+ for their layout.

h6. Choosing Layouts at Runtime

You can use a symbol to defer the choice of layout until a request is processed:

<ruby>
class ProductsController < ApplicationController
  layout :products_layout

  def show
    @product = Product.find(params[:id])
  end

  private
    def products_layout
      @current_user.special? ? "special" : "products"
    end

end
</ruby>

Now, if the current user is a special user, they'll get a special layout when viewing a product. You can even use an inline method to determine the layout:

You can also decide the layout by passing a Proc object, the block you give the Proc will be given the +controller+ instance, so you can make decisions based on the current request.  For example:

<ruby>
class ProductsController < ApplicationController
  layout Proc.new { |controller| controller.request.xhr? ? 'popup' : 'application' }
end
</ruby>

h6. Conditional Layouts

Layouts specified at the controller level support +:only+ and +:except+ options that take either a method name or an array of method names which correspond to method names within the controller:

<ruby>
class ProductsController < ApplicationController
  layout "product", :except => [:index, :rss]
end
</ruby>

With this declaration, the +product+ layout would be used for everything but the +rss+ and +index+ methods.

h6. Layout Inheritance

Layouts are shared downwards in the hierarchy, and more specific layouts always override more general ones. For example:

* +application_controller.rb+

<ruby>
class ApplicationController < ActionController::Base
  layout "main"
end
</ruby>

* +posts_controller.rb+

<ruby>
class PostsController < ApplicationController
end
</ruby>

* +special_posts_controller.rb+

<ruby>
class SpecialPostsController < PostsController
  layout "special"
end
</ruby>

* +old_posts_controller.rb+

<ruby>
class OldPostsController < SpecialPostsController
  layout nil

  def show
    @post = Post.find(params[:id])
  end

  def index
    @old_posts = Post.older
    render :layout => "old"
  end
  # ...
end
</ruby>

In this application:

* In general, views will be rendered in the +main+ layout
* +PostsController#index+ will use the +main+ layout
* +SpecialPostsController#index+ will use the +special+ layout
* +OldPostsController#show+ will use no layout at all
* +OldPostsController#index+ will use the +old+ layout

h5. Avoiding Double Render Errors

Sooner or later, most Rails developers will see the error message "Can only render or redirect once per action". While this is annoying, it's relatively easy to fix. Usually it happens because of a fundamental misunderstanding of the way that +render+ works.

For example, here's some code that will trigger this error:

<ruby>
def show
  @book = Book.find(params[:id])
  if @book.special?
    render :action => "special_show"
  end
  render :action => "regular_show"
end
</ruby>

If +@book.special?+ evaluates to +true+, Rails will start the rendering process to dump the +@book+ variable into the +special_show+ view. But this will _not_ stop the rest of the code in the +show+ action from running, and when Rails hits the end of the action, it will start to render the +regular_show+ view - and throw an error. The solution is simple: make sure that you have only one call to +render+ or +redirect+ in a single code path. One thing that can help is +and return+. Here's a patched version of the method:

<ruby>
def show
  @book = Book.find(params[:id])
  if @book.special?
    render :action => "special_show" and return
  end
  render :action => "regular_show"
end
</ruby>

Make sure you use +and return+ and not +&amp;&amp; return+ because while the former will work, the latter will not due to operator precedence in the Ruby Language.

Note that the implicit render done by ActionController detects if +render+ has been called, and thus avoids this error. Therefore, the following will work without errors:

<ruby>
def show
  @book = Book.find(params[:id])
  if @book.special?
    render :action => "special_show"
  end
end
</ruby>

This will render a book with +special?+ set with the +special_show+ template, while other books will render with the default +show+ template.

h4. Using +redirect_to+

Another way to handle returning responses to an HTTP request is with +redirect_to+. As you've seen, +render+ tells Rails which view (or other asset) to use in constructing a response. The +redirect_to+ method does something completely different: it tells the browser to send a new request for a different URL. For example, you could redirect from wherever you are in your code to the index of photos in your application with this call:

<ruby>
redirect_to photos_path
</ruby>

You can use +redirect_to+ with any arguments that you could use with +link_to+ or +url_for+. In addition, there's a special redirect that sends the user back to the page they just came from:

<ruby>
redirect_to :back
</ruby>

h5. Getting a Different Redirect Status Code

Rails uses HTTP status code 302 (temporary redirect) when you call +redirect_to+. If you'd like to use a different status code (perhaps 301, permanent redirect), you can do so by using the +:status+ option:

<ruby>
redirect_to photos_path, :status => 301
</ruby>

Just like the +:status+ option for +render+, +:status+ for +redirect_to+ accepts both numeric and symbolic header designations.

h5. The Difference Between +render+ and +redirect_to+

Sometimes inexperienced developers conceive of +redirect_to+ as a sort of +goto+ command, moving execution from one place to another in your Rails code. This is _not_ correct. Your code stops running and waits for a new request for the browser. It just happens that you've told the browser what request it should make next, by sending back an HTTP 302 status code.

Consider these actions to see the difference:

<ruby>
def index
  @books = Book.all
end

def show
  @book = Book.find_by_id(params[:id])
  if @book.nil?
    render :action => "index"
  end
end
</ruby>

With the code in this form, there will likely be a problem if the +@book+ variable is +nil+. Remember, a +render :action+ doesn't run any code in the target action, so nothing will set up the +@books+ variable that the +index+ view is presumably depending on. One way to fix this is to redirect instead of rendering:

<ruby>
def index
  @books = Book.all
end

def show
  @book = Book.find_by_id(params[:id])
  if @book.nil?
    redirect_to :action => :index
  end
end
</ruby>

With this code, the browser will make a fresh request for the index page, the code in the +index+ method will run, and all will be well.

The only downside to this code, is that it requires a round trip to the browser, the browser requested the show action with +/books/1+ and the controller finds that there are no books, so the controller sends out a 302 redirect response to the browser telling it to go to +/books/+, the browser complies and sends a new request back to the controller asking now for the +index+ action, the controller then gets all the books in the database and renders the index template, sending it back down to the browser which then shows it on your screen.

While in a small app, this added latency might not be a problem, it is something to think about when speed of response is of the essence.  One way to handle this double request (though a contrived example) could be:

<ruby>
def index
  @books = Book.all
end

def show
  @book = Book.find_by_id(params[:id])
  if @book.nil?
    @books = Book.all
    render "index", :alert => 'Your book was not found!'
  end
end
</ruby>

Which would detect that there are no books, populate the +@books+ instance variable with all the books in the database and then directly render the +index.html.erb+ template returning it to the browser with a flash alert message telling the user what happened.

h4. Using +head+ To Build Header-Only Responses

The +head+ method exists to let you send back responses to the browser that have only headers. It provides a more obvious alternative to calling +render :nothing+. The +head+ method takes one response, which is interpreted as a hash of header names and values. For example, you can return only an error header:

<ruby>
head :bad_request
</ruby>

Which would produce the following header:

<shell>
HTTP/1.1 400 Bad Request
Connection: close
Date: Sun, 24 Jan 2010 12:15:53 GMT
Transfer-Encoding: chunked
Content-Type: text/html; charset=utf-8
X-Runtime: 0.013483
Set-Cookie: _blog_session=...snip...; path=/; HttpOnly
Cache-Control: no-cache
</shell>

Or you can use other HTTP headers to convey additional information:

<ruby>
head :created, :location => photo_path(@photo)
</ruby>

Which would produce:

<shell>
HTTP/1.1 201 Created
Connection: close
Date: Sun, 24 Jan 2010 12:16:44 GMT
Transfer-Encoding: chunked
Location: /photos/1
Content-Type: text/html; charset=utf-8
X-Runtime: 0.083496
Set-Cookie: _blog_session=...snip...; path=/; HttpOnly
Cache-Control: no-cache
</shell>

h3. Structuring Layouts

When Rails renders a view as a response, it does so by combining the view with the current layout (using the rules for finding the current layout that were covered earlier in this guide). Within a layout, you have access to three tools for combining different bits of output to form the overall response:

* Asset tags
* +yield+ and +content_for+
* Partials

I'll discuss each of these in turn.

h4. Asset Tags

Asset tags provide methods for generating HTML that links views to assets like images, videos, audio, JavaScript, stylesheets, and feeds. There are six types of include tag:

* +auto_discovery_link_tag+
* +javascript_include_tag+
* +stylesheet_link_tag+
* +image_tag+
* +video_tag+
* +audio_tag+

You can use these tags in layouts or other views, although the tags other than +image_tag+ are most commonly used in the +&lt;head&gt;+ section of a layout.

WARNING: The asset tags do _not_ verify the existence of the assets at the specified locations; they simply assume that you know what you're doing and generate the link.

h5. Linking to Feeds with +auto_discovery_link_tag+

The +auto_discovery_link_tag+ helper builds HTML that most browsers and newsreaders can use to detect the presences of RSS or ATOM feeds. It takes the type of the link (+:rss+ or +:atom+), a hash of options that are passed through to url_for, and a hash of options for the tag:

<erb>
<%= auto_discovery_link_tag(:rss, {:action => "feed"},
  {:title => "RSS Feed"}) %>
</erb>

There are three tag options available for +auto_discovery_link_tag+:

* +:rel+ specifies the +rel+ value in the link (defaults to "alternate")
* +:type+ specifies an explicit MIME type. Rails will generate an appropriate MIME type automatically
* +:title+ specifies the title of the link

h5. Linking to Javascript Files with +javascript_include_tag+

The +javascript_include_tag+ helper returns an HTML +script+ tag for each source provided. Rails looks in +public/javascripts+ for these files by default, but you can specify a full path relative to the document root, or a URL, if you prefer. For example, to include +public/javascripts/main.js+:

<erb>
<%= javascript_include_tag "main" %>
</erb>

To include +public/javascripts/main.js+ and +public/javascripts/columns.js+:

<erb>
<%= javascript_include_tag "main", "columns" %>
</erb>

To include +public/javascripts/main.js+ and +public/photos/columns.js+:

<erb>
<%= javascript_include_tag "main", "/photos/columns" %>
</erb>

To include +http://example.com/main.js+:

<erb>
<%= javascript_include_tag "http://example.com/main.js" %>
</erb>

The +defaults+ option loads the Prototype and Scriptaculous libraries:

<erb>
<%= javascript_include_tag :defaults %>
</erb>

The +all+ option loads every JavaScript file in +public/javascripts+, starting with the Prototype and Scriptaculous libraries:

<erb>
<%= javascript_include_tag :all %>
</erb>

You can supply the +:recursive+ option to load files in subfolders of +public/javascripts+ as well:

<erb>
<%= javascript_include_tag :all, :recursive => true %>
</erb>

If you're loading multiple JavaScript files, you can create a better user experience by combining multiple files into a single download. To make this happen in production, specify +:cache => true+ in your +javascript_include_tag+:

<erb>
<%= javascript_include_tag "main", "columns", :cache => true %>
</erb>

By default, the combined file will be delivered as +javascripts/all.js+. You can specify a location for the cached asset file instead:

<erb>
<%= javascript_include_tag "main", "columns",
  :cache => 'cache/main/display' %>
</erb>

You can even use dynamic paths such as +cache/#{current_site}/main/display+.

h5. Linking to CSS Files with +stylesheet_link_tag+

The +stylesheet_link_tag+ helper returns an HTML +&lt;link&gt;+ tag for each source provided. Rails looks in +public/stylesheets+ for these files by default, but you can specify a full path relative to the document root, or a URL, if you prefer. For example, to include +public/stylesheets/main.css+:

<erb>
<%= stylesheet_link_tag "main" %>
</erb>

To include +public/stylesheets/main.css+ and +public/stylesheets/columns.css+:

<erb>
<%= stylesheet_link_tag "main", "columns" %>
</erb>

To include +public/stylesheets/main.css+ and +public/photos/columns.css+:

<erb>
<%= stylesheet_link_tag "main", "/photos/columns" %>
</erb>

To include +http://example.com/main.css+:

<erb>
<%= stylesheet_link_tag "http://example.com/main.css" %>
</erb>

By default, +stylesheet_link_tag+ creates links with +media="screen" rel="stylesheet" type="text/css"+. You can override any of these defaults by specifying an appropriate option (+:media+, +:rel+, or +:type+):

<erb>
<%= stylesheet_link_tag "main_print", :media => "print" %>
</erb>

The +all+ option links every CSS file in +public/stylesheets+:

<erb>
<%= stylesheet_link_tag :all %>
</erb>

You can supply the +:recursive+ option to link files in subfolders of +public/stylesheets+ as well:

<erb>
<%= stylesheet_link_tag :all, :recursive => true %>
</erb>

If you're loading multiple CSS files, you can create a better user experience by combining multiple files into a single download. To make this happen in production, specify +:cache => true+ in your +stylesheet_link_tag+:

<erb>
<%= stylesheet_link_tag "main", "columns", :cache => true %>
</erb>

By default, the combined file will be delivered as +stylesheets/all.css+. You can specify a location for the cached asset file instead:

<erb>
<%= stylesheet_link_tag "main", "columns",
  :cache => 'cache/main/display' %>
</erb>

You can even use dynamic paths such as +cache/#{current_site}/main/display+.

h5. Linking to Images with +image_tag+

The +image_tag+ helper builds an HTML +&lt;img /&gt;+ tag to the specified file. By default, files are loaded from +public/images+.

WARNING: Note that you must specify the extension of the image. Previous versions of Rails would allow you to just use the image name and would append +.png+ if no extension was given but Rails 3.0 does not.

<erb>
<%= image_tag "header.png" %>
</erb>

You can supply a path to the image if you like:

<erb>
<%= image_tag "icons/delete.gif" %>
</erb>

You can supply a hash of additional HTML options:

<erb>
<%= image_tag "icons/delete.gif", {:height => 45} %>
</erb>

You can also supply an alternate image to show on mouseover:

<erb>
<%= image_tag "home.gif", :onmouseover => "menu/home_highlight.gif" %>
</erb>

Or alternate text if the user has rendering images turned off in their browser, if you do not specify an explicit alt tag, it defaults to the file name of the file, capitalized and with no extension, for example, these two image tags would return the same code:

<erb>
<%= image_tag "home.gif" %>
<%= image_tag "home.gif", :alt => "Home" %>
</erb>

You can also specify a special size tag, in the format "{width}x{height}":

<erb>
<%= image_tag "home.gif", :size => "50x20" %>
</erb>

In addition to the above special tags, you can supply a final hash of standard HTML options, such as +:class+, +:id+ or +:name+:

<erb>
<%= image_tag "home.gif", :alt => "Go Home",
                          :id => "HomeImage",
                          :class => 'nav_bar' %>
</erb>

h5. Linking to Videos with +video_tag+

The +video_tag+ helper builds an HTML 5 +&lt;video&gt;+ tag to the specified file. By default, files are loaded from +public/videos+.

<erb>
<%= video_tag "movie.ogg" %>
</erb>

Produces

<erb>
<video src="/videos/movie.ogg" />
</erb>

Like an +image_tag+ you can supply a path, either absolute, or relative to the +public/videos+ directory.  Additionally you can specify the +:size => "#{width}x#{height}"+ option just like an +image_tag+.  Video tags can also have any of the HTML options specified at the end (+id+, +class+ et al).

The video tag also supports all of the +&lt;video&gt;+ HTML options through the HTML options hash, including:

* +:poster => 'image_name.png'+, provides an image to put in place of the video before it starts playing.
* +:autoplay => true+, starts playing the video on page load.
* +:loop => true+, loops the video once it gets to the end.
* +:controls => true+, provides browser supplied controls for the user to interact with the video.
* +:autobuffer => true+, the video will pre load the file for the user on page load.

You can also specify multiple videos to play by passing an array of videos to the +video_tag+:

<erb>
<%= video_tag ["trailer.ogg", "movie.ogg"] %>
</erb>

This will produce:

<erb>
<video><source src="trailer.ogg" /><source src="movie.ogg" /></video>
</erb>

h5. Linking to Audio files with +audio_tag+

The +audio_tag+ helper builds an HTML 5 +&lt;audio&gt;+ tag to the specified file. By default, files are loaded from +public/audios+.

<erb>
<%= audio_tag "music.mp3" %>
</erb>

You can supply a path to the audio file if you like:

<erb>
<%= audio_tag "music/first_song.mp3" %>
</erb>

You can also supply a hash of additional options, such as +:id+, +:class+ etc.

Like the +video_tag+, the +audio_tag+ has special options:

* +:autoplay => true+, starts playing the audio on page load
* +:controls => true+, provides browser supplied controls for the user to interact with the audio.
* +:autobuffer => true+, the audio will pre load the file for the user on page load.

h4. Understanding +yield+

Within the context of a layout, +yield+ identifies a section where content from the view should be inserted. The simplest way to use this is to have a single +yield+, into which the entire contents of the view currently being rendered is inserted:

<erb>
<html>
  <head>
  </head>
  <body>
  <%= yield %>
  </body>
</html>
</erb>

You can also create a layout with multiple yielding regions:

<erb>
<html>
  <head>
  <%= yield :head %>
  </head>
  <body>
  <%= yield %>
  </body>
</html>
</erb>

The main body of the view will always render into the unnamed +yield+. To render content into a named +yield+, you use the +content_for+ method.

h4. Using +content_for+

The +content_for+ method allows you to insert content into a +yield+ block in your layout. You only use +content_for+ to insert content in named yields. For example, this view would work with the layout that you just saw:

<erb>
<% content_for :head do %>
  <title>A simple page</title>
<% end %>

<p>Hello, Rails!</p>
</erb>

The result of rendering this page into the supplied layout would be this HTML:

<erb>
<html>
  <head>
  <title>A simple page</title>
  </head>
  <body>
  <p>Hello, Rails!</p>
  </body>
</html>
</erb>

The +content_for+ method is very helpful when your layout contains distinct regions such as sidebars and footers that should get their own blocks of content inserted. It's also useful for inserting tags that load page-specific JavaScript or css files into the header of an otherwise generic layout.

h4. Using Partials

Partial templates - usually just called "partials" - are another device for breaking apart the rendering process into more manageable chunks. With a partial, you can move the code for rendering a particular piece of a response to its own file.

h5. Naming Partials

To render a partial as part of a view, you use the +render+ method within the view:

<ruby>
<%= render "menu" %>
</ruby>

This will render a file named +_menu.html.erb+ at that point within the view being rendered. Note the leading underscore character: partials are named with a leading underscore to distinguish them from regular views, even though they are referred to without the underscore. This holds true even when you're pulling in a partial from another folder:

<ruby>
<%= render "shared/menu" %>
</ruby>

That code will pull in the partial from +app/views/shared/_menu.html.erb+.

h5. Using Partials to Simplify Views

One way to use partials is to treat them as the equivalent of subroutines: as a way to move details out of a view so that you can grasp what's going on more easily. For example, you might have a view that looked like this:

<erb>
<%= render "shared/ad_banner" %>

<h1>Products</h1>

<p>Here are a few of our fine products:</p>
...

<%= render "shared/footer" %>
</erb>

Here, the +_ad_banner.html.erb+ and +_footer.html.erb+ partials could contain content that is shared among many pages in your application. You don't need to see the details of these sections when you're concentrating on a particular page.

TIP: For content that is shared among all pages in your application, you can use partials directly from layouts.

h5. Partial Layouts

A partial can use its own layout file, just as a view can use a layout. For example, you might call a partial like this:

<erb>
<%= render "link_area", :layout => "graybar" %>
</erb>

This would look for a partial named +_link_area.html.erb+ and render it using the layout +_graybar.html.erb+. Note that layouts for partials follow the same leading-underscore naming as regular partials, and are placed in the same folder with the partial that they belong to (not in the master +layouts+ folder).

h5. Passing Local Variables

You can also pass local variables into partials, making them even more powerful and flexible. For example, you can use this technique to reduce duplication between new and edit pages, while still keeping a bit of distinct content:

* +new.html.erb+

<erb>
<h1>New zone</h1>
<%= error_messages_for :zone %>
<%= render :partial => "form", :locals => { :zone => @zone } %>
</erb>

* +edit.html.erb+

<erb>
<h1>Editing zone</h1>
<%= error_messages_for :zone %>
<%= render :partial => "form", :locals => { :zone => @zone } %>
</erb>

* +_form.html.erb+

<erb>
<%= form_for(zone) do |f| %>
  <p>
    <b>Zone name</b><br />
    <%= f.text_field :name %>
  </p>
  <p>
    <%= f.submit %>
  </p>
<% end %>
</erb>

Although the same partial will be rendered into both views, Action View's submit helper will return "Create Zone" for the new action and "Update Zone" for the edit action.

Every partial also has a local variable with the same name as the partial (minus the underscore). You can pass an object in to this local variable via the +:object+ option:

<erb>
<%= render :partial => "customer", :object => @new_customer %>
</erb>

Within the +customer+ partial, the +customer+ variable will refer to +@new_customer+ from the parent view.

WARNING: In previous versions of Rails, the default local variable would look for an instance variable with the same name as the partial in the parent. This behavior was deprecated in 2.3 and has been removed in Rails 3.0.

If you have an instance of a model to render into a partial, you can use a shorthand syntax:

<erb>
<%= render @customer %>
</erb>

Assuming that the +@customer+ instance variable contains an instance of the +Customer+ model, this will use +_customer.html.erb+ to render it and will pass the local variable +customer+ into the partial which will refer to the +@customer+ instance variable in the parent view.

h5. Rendering Collections

Partials are very useful in rendering collections. When you pass a collection to a partial via the +:collection+ option, the partial will be inserted once for each member in the collection:

* +index.html.erb+

<erb>
<h1>Products</h1>
<%= render :partial => "product", :collection => @products %>
</erb>

* +_product.html.erb+

<erb>
<p>Product Name: <%= product.name %></p>
</erb>

When a partial is called with a pluralized collection, then the individual instances of the partial have access to the member of the collection being rendered via a variable named after the partial. In this case, the partial is +_product+, and within the +_product+ partial, you can refer to +product+ to get the instance that is being rendered.

In Rails 3.0, there is also a shorthand for this. Assuming +@products+ is a collection of +product+ instances, you can simply write this in the +index.html.erb+:

<erb>
<h1>Products</h1>
<%= render @products %>
</erb>

To produce the same result.

Rails determines the name of the partial to use by looking at the model name in the collection. In fact, you can even create a heterogeneous collection and render it this way, and Rails will choose the proper partial for each member of the collection:

* +index.html.erb+

<erb>
<h1>Contacts</h1>
<%= render [customer1, employee1, customer2, employee2] %>
</erb>

* +customers/_customer.html.erb+

<erb>
<p>Customer: <%= customer.name %></p>
</erb>

* +employees/_employee.html.erb+

<erb>
<p>Employee: <%= employee.name %></p>
</erb>

In this case, Rails will use the customer or employee partials as appropriate for each member of the collection.

h5. Local Variables

To use a custom local variable name within the partial, specify the +:as+ option in the call to the partial:

<erb>
<%= render :partial => "product", :collection => @products, :as => :item %>
</erb>

With this change, you can access an instance of the +@products+ collection as the +item+ local variable within the partial.

You can also pass in arbitrary local variables to any partial you are rendering with the +:locals => {}+ option:

<erb>
<%= render :partial => 'products', :collection => @products,
           :as => :item, :locals => {:title => "Products Page"} %>
</erb>

Would render a partial +_products.html.erb+ once for each instance of +product+ in the +@products+ instance variable passing the instance to the partial as a local variable called +item+ and to each partial, make the local variable +title+ available with the value +Products Page+.

TIP: Rails also makes a counter variable available within a partial called by the collection, named after the member of the collection followed by +_counter+. For example, if you're rendering +@products+, within the partial you can refer to +product_counter+ to tell you how many times the partial has been rendered.  This does not work in conjunction with the +:as => :value+ option.

You can also specify a second partial to be rendered between instances of the main partial by using the +:spacer_template+ option:

h5. Spacer Templates

<erb>
<%= render @products, :spacer_template => "product_ruler" %>
</erb>

Rails will render the +_product_ruler+ partial (with no data passed in to it) between each pair of +_product+ partials.

h4. Using Nested Layouts

You may find that your application requires a layout that differs slightly from your regular application layout to support one particular controller. Rather than repeating the main layout and editing it, you can accomplish this by using nested layouts (sometimes called sub-templates). Here's an example:

Suppose you have the following +ApplicationController+ layout:

* +app/views/layouts/application.html.erb+

<erb>
<html>
<head>
  <title><%= @page_title or 'Page Title' %></title>
  <%= stylesheet_link_tag 'layout' %>
  <style type="text/css"><%= yield :stylesheets %></style>
</head>
<body>
  <div id="top_menu">Top menu items here</div>
  <div id="menu">Menu items here</div>
  <div id="content"><%= content_for?(:content) ? yield(:content) : yield %></div>
</body>
</html>
</erb>

On pages generated by +NewsController+, you want to hide the top menu and add a right menu:

* +app/views/layouts/news.html.erb+

<erb>
<% content_for :stylesheets do %>
  #top_menu {display: none}
  #right_menu {float: right; background-color: yellow; color: black}
<% end %>
<% content_for :content do %>
  <div id="right_menu">Right menu items here</div>
  <%= yield(:news_content) or yield %>
<% end %>
<%= render :template => 'layouts/application' %>
</erb>

That's it. The News views will use the new layout, hiding the top menu and adding a new right menu inside the "content" div.

There are several ways of getting similar results with different sub-templating schemes using this technique. Note that there is no limit in nesting levels. One can use the +ActionView::render+ method via +render :template => 'layouts/news'+ to base a new layout on the News layout. If you are sure you will not subtemplate the +News+ layout, you can replace the +yield(:news_content) or yield+ with simply +yield+.

h3. Changelog

"Lighthouse ticket":http://rails.lighthouseapp.com/projects/16213-rails-guides/tickets/15

* April 4, 2010: Fixed document to validate XHTML 1.0 Strict. "Jaime Iniesta":http://jaimeiniesta.com
* January 25, 2010: Rails 3.0 Update by "Mikel Lindsaar":credits.html#raasdnil
* December 27, 2008: Merge patch from Rodrigo Rosenfeld Rosas covering subtemplates
* December 27, 2008: Information on new rendering defaults by "Mike Gunderloy":credits.html#mgunderloy
* November 9, 2008: Added partial collection counter by "Mike Gunderloy":credits.html#mgunderloy
* November 1, 2008: Added +:js+ option for +render+ by "Mike Gunderloy":credits.html#mgunderloy
* October 16, 2008: Ready for publication by "Mike Gunderloy":credits.html#mgunderloy
* October 4, 2008: Additional info on partials (+:object+, +:as+, and +:spacer_template+) by "Mike Gunderloy":credits.html#mgunderloy (not yet approved for publication)
* September 28, 2008: First draft by "Mike Gunderloy":credits.html#mgunderloy (not yet approved for publication)