-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy pathactive_record_migrations.html
1129 lines (1044 loc) · 58.5 KB
/
active_record_migrations.html
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
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Active Record Migrations — Ruby on Rails Guides</title>
<link rel="stylesheet" type="text/css" href="stylesheets/style.css" data-turbolinks-track="reload">
<link rel="stylesheet" type="text/css" href="stylesheets/print.css" media="print">
<link rel="stylesheet" type="text/css" href="stylesheets/syntaxhighlighter/shCore.css" data-turbolinks-track="reload">
<link rel="stylesheet" type="text/css" href="stylesheets/syntaxhighlighter/shThemeRailsGuides.css" data-turbolinks-track="reload">
<link rel="stylesheet" type="text/css" href="stylesheets/fixes.css" data-turbolinks-track="reload">
<link href="images/favicon.ico" rel="shortcut icon" type="image/x-icon" />
<script src="javascripts/syntaxhighlighter.js" data-turbolinks-track="reload"></script>
<script src="javascripts/turbolinks.js" data-turbolinks-track="reload"></script>
<script src="javascripts/guides.js" data-turbolinks-track="reload"></script>
<script src="javascripts/responsive-tables.js" data-turbolinks-track="reload"></script>
</head>
<body class="guide">
<div>
<img src="images/edge_badge.png" alt="edge-badge" id="edge-badge" />
</div>
<div id="topNav">
<div class="wrapper">
<strong class="more-info-label">Veja mais em <a href="http://rubyonrails.org/">rubyonrails.org:</a> </strong>
<span class="red-button more-info-button">
Mais Ruby on Rails
</span>
<ul class="more-info-links s-hidden">
<li class="more-info"><a href="https://weblog.rubyonrails.org/">Blog</a></li>
<li class="more-info"><a href="https://guides.rubyonrails.org/">Guia</a></li>
<li class="more-info"><a href="http://api.rubyonrails.org/">API</a></li>
<li class="more-info"><a href="https://stackoverflow.com/questions/tagged/ruby-on-rails">Pedir ajuda</a></li>
<li class="more-info"><a href="https://github.com/rails/rails">Contribuir no GitHub</a></li>
</ul>
</div>
</div>
<div id="header">
<div class="wrapper clearfix">
<h1><a href="index.html" title="Return to home page">Guides.rubyonrails.org</a></h1>
<ul class="nav">
<li><a class="nav-item" href="index.html">Home</a></li>
<li class="guides-index guides-index-large">
<a href="index.html" id="guidesMenu" class="guides-index-item nav-item">Guias</a>
<div id="guides" class="clearfix" style="display: none;">
<hr />
<div class="guides-section-container">
<div class="guides-section">
<dt>Comece Aqui</dt>
</div>
<div class="guides-section">
<dt>Models</dt>
</div>
<div class="guides-section">
<dt>Views</dt>
</div>
<div class="guides-section">
<dt>Controllers</dt>
</div>
<div class="guides-section">
<dt>Other Components</dt>
</div>
<div class="guides-section">
<dt>Digging Deeper</dt>
</div>
<div class="guides-section">
<dt>Extending Rails</dt>
<dd><a href="rails_on_rack.html">Rails on Rack</a></dd>
<dd><a href="generators.html">Creating and Customizing Rails Generators & Templates</a></dd>
</div>
<div class="guides-section">
<dt>Contributions</dt>
<dd><a href="contributing_to_ruby_on_rails.html">Contributing to Ruby on Rails</a></dd>
<dd><a href="api_documentation_guidelines.html">API Documentation Guidelines</a></dd>
<dd><a href="ruby_on_rails_guides_guidelines.html">Guides Guidelines</a></dd>
</div>
<div class="guides-section">
<dt>Policies</dt>
<dd><a href="maintenance_policy.html">Maintenance Policy</a></dd>
</div>
<div class="guides-section">
<dt>Release Notes</dt>
<dd><a href="upgrading_ruby_on_rails.html">Upgrading Ruby on Rails</a></dd>
<dd><a href="5_2_release_notes.html">Version 5.2 - April 2018</a></dd>
<dd><a href="5_1_release_notes.html">Version 5.1 - April 2017</a></dd>
<dd><a href="5_0_release_notes.html">Version 5.0 - June 2016</a></dd>
<dd><a href="4_2_release_notes.html">Version 4.2 - December 2014</a></dd>
<dd><a href="4_1_release_notes.html">Version 4.1 - April 2014</a></dd>
<dd><a href="4_0_release_notes.html">Version 4.0 - June 2013</a></dd>
<dd><a href="3_2_release_notes.html">Version 3.2 - January 2012</a></dd>
<dd><a href="3_1_release_notes.html">Version 3.1 - August 2011</a></dd>
<dd><a href="3_0_release_notes.html">Version 3.0 - August 2010</a></dd>
<dd><a href="2_3_release_notes.html">Version 2.3 - March 2009</a></dd>
<dd><a href="2_2_release_notes.html">Version 2.2 - November 2008</a></dd>
</div>
</div>
</div>
</li>
<li><a class="nav-item" href="contributing_to_ruby_on_rails.html">Contribua</a></li>
<li class="guides-index guides-index-small">
<select class="guides-index-item nav-item">
<option value="index.html">Guias</option>
<optgroup label="Comece Aqui">
</optgroup>
<optgroup label="Models">
</optgroup>
<optgroup label="Views">
</optgroup>
<optgroup label="Controllers">
</optgroup>
<optgroup label="Other Components">
</optgroup>
<optgroup label="Digging Deeper">
</optgroup>
<optgroup label="Extending Rails">
<option value="rails_on_rack.html">Rails on Rack</option>
<option value="generators.html">Creating and Customizing Rails Generators & Templates</option>
</optgroup>
<optgroup label="Contributions">
<option value="contributing_to_ruby_on_rails.html">Contributing to Ruby on Rails</option>
<option value="api_documentation_guidelines.html">API Documentation Guidelines</option>
<option value="ruby_on_rails_guides_guidelines.html">Guides Guidelines</option>
</optgroup>
<optgroup label="Policies">
<option value="maintenance_policy.html">Maintenance Policy</option>
</optgroup>
<optgroup label="Release Notes">
<option value="upgrading_ruby_on_rails.html">Upgrading Ruby on Rails</option>
<option value="5_2_release_notes.html">Version 5.2 - April 2018</option>
<option value="5_1_release_notes.html">Version 5.1 - April 2017</option>
<option value="5_0_release_notes.html">Version 5.0 - June 2016</option>
<option value="4_2_release_notes.html">Version 4.2 - December 2014</option>
<option value="4_1_release_notes.html">Version 4.1 - April 2014</option>
<option value="4_0_release_notes.html">Version 4.0 - June 2013</option>
<option value="3_2_release_notes.html">Version 3.2 - January 2012</option>
<option value="3_1_release_notes.html">Version 3.1 - August 2011</option>
<option value="3_0_release_notes.html">Version 3.0 - August 2010</option>
<option value="2_3_release_notes.html">Version 2.3 - March 2009</option>
<option value="2_2_release_notes.html">Version 2.2 - November 2008</option>
</optgroup>
</select>
</li>
</ul>
</div>
</div>
<hr class="hide" />
<div id="feature">
<div class="wrapper">
<h2>Active Record Migrations</h2><p>Migrations are a feature of Active Record that allows you to evolve your
database schema over time. Rather than write schema modifications in pure SQL,
migrations allow you to use an easy Ruby DSL to describe changes to your
tables.</p><p>After reading this guide, you will know:</p>
<ul>
<li>The generators you can use to create them.</li>
<li>The methods Active Record provides to manipulate your database.</li>
<li>The rails commands that manipulate migrations and your schema.</li>
<li>How migrations relate to <code>schema.rb</code>.</li>
</ul>
<div id="subCol">
<h3 class="chapter"><img src="images/chapters_icon.gif" alt="" />Chapters</h3>
<ol class="chapters">
<li><a href="#migration-overview">Migration Overview</a></li>
<li>
<a href="#creating-a-migration">Creating a Migration</a>
<ul>
<li><a href="#creating-a-standalone-migration">Creating a Standalone Migration</a></li>
<li><a href="#model-generators">Model Generators</a></li>
<li><a href="#passing-modifiers">Passing Modifiers</a></li>
</ul>
</li>
<li>
<a href="#writing-a-migration">Writing a Migration</a>
<ul>
<li><a href="#creating-a-table">Creating a Table</a></li>
<li><a href="#creating-a-join-table">Creating a Join Table</a></li>
<li><a href="#changing-tables">Changing Tables</a></li>
<li><a href="#changing-columns">Changing Columns</a></li>
<li><a href="#column-modifiers">Column Modifiers</a></li>
<li><a href="#foreign-keys">Foreign Keys</a></li>
<li><a href="#when-helpers-aren-t-enough">When Helpers aren't Enough</a></li>
<li><a href="#using-the-change-method">Using the <code>change</code> Method</a></li>
<li><a href="#using-reversible">Using <code>reversible</code></a></li>
<li><a href="#using-the-up-down-methods">Using the <code>up</code>/<code>down</code> Methods</a></li>
<li><a href="#reverting-previous-migrations">Reverting Previous Migrations</a></li>
</ul>
</li>
<li>
<a href="#running-migrations">Running Migrations</a>
<ul>
<li><a href="#rolling-back">Rolling Back</a></li>
<li><a href="#setup-the-database">Setup the Database</a></li>
<li><a href="#resetting-the-database">Resetting the Database</a></li>
<li><a href="#running-specific-migrations">Running Specific Migrations</a></li>
<li><a href="#running-migrations-in-different-environments">Running Migrations in Different Environments</a></li>
<li><a href="#changing-the-output-of-running-migrations">Changing the Output of Running Migrations</a></li>
</ul>
</li>
<li><a href="#changing-existing-migrations">Changing Existing Migrations</a></li>
<li>
<a href="#schema-dumping-and-you">Schema Dumping and You</a>
<ul>
<li><a href="#what-are-schema-files-for-questionmark">What are Schema Files for?</a></li>
<li><a href="#types-of-schema-dumps">Types of Schema Dumps</a></li>
<li><a href="#schema-dumps-and-source-control">Schema Dumps and Source Control</a></li>
</ul>
</li>
<li><a href="#active-record-and-referential-integrity">Active Record and Referential Integrity</a></li>
<li><a href="#migrations-and-seed-data">Migrations and Seed Data</a></li>
<li><a href="#old-migrations">Old Migrations</a></li>
</ol>
</div>
</div>
</div>
<div id="container">
<div class="wrapper">
<div id="mainCol">
<h3 id="migration-overview"><a class="anchorlink" href="#migration-overview">1 Migration Overview</a></h3><p>Migrations are a convenient way to
<a href="https://en.wikipedia.org/wiki/Schema_migration">alter your database schema over time</a>
in a consistent and easy way. They use a Ruby DSL so that you don't have to
write SQL by hand, allowing your schema and changes to be database independent.</p><p>You can think of each migration as being a new 'version' of the database. A
schema starts off with nothing in it, and each migration modifies it to add or
remove tables, columns, or entries. Active Record knows how to update your
schema along this timeline, bringing it from whatever point it is in the
history to the latest version. Active Record will also update your
<code>db/schema.rb</code> file to match the up-to-date structure of your database.</p><p>Here's an example of a migration:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class CreateProducts < ActiveRecord::Migration[5.0]
def change
create_table :products do |t|
t.string :name
t.text :description
t.timestamps
end
end
end
</pre>
</div>
<p>This migration adds a table called <code>products</code> with a string column called
<code>name</code> and a text column called <code>description</code>. A primary key column called <code>id</code>
will also be added implicitly, as it's the default primary key for all Active
Record models. The <code>timestamps</code> macro adds two columns, <code>created_at</code> and
<code>updated_at</code>. These special columns are automatically managed by Active Record
if they exist.</p><p>Note that we define the change that we want to happen moving forward in time.
Before this migration is run, there will be no table. After, the table will
exist. Active Record knows how to reverse this migration as well: if we roll
this migration back, it will remove the table.</p><p>On databases that support transactions with statements that change the schema,
migrations are wrapped in a transaction. If the database does not support this
then when a migration fails the parts of it that succeeded will not be rolled
back. You will have to rollback the changes that were made by hand.</p><div class="note"><p>There are certain queries that can't run inside a transaction. If your
adapter supports DDL transactions you can use <code>disable_ddl_transaction!</code> to
disable them for a single migration.</p></div><p>If you wish for a migration to do something that Active Record doesn't know how
to reverse, you can use <code>reversible</code>:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class ChangeProductsPrice < ActiveRecord::Migration[5.0]
def change
reversible do |dir|
change_table :products do |t|
dir.up { t.change :price, :string }
dir.down { t.change :price, :integer }
end
end
end
end
</pre>
</div>
<p>Alternatively, you can use <code>up</code> and <code>down</code> instead of <code>change</code>:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class ChangeProductsPrice < ActiveRecord::Migration[5.0]
def up
change_table :products do |t|
t.change :price, :string
end
end
def down
change_table :products do |t|
t.change :price, :integer
end
end
end
</pre>
</div>
<h3 id="creating-a-migration"><a class="anchorlink" href="#creating-a-migration">2 Creating a Migration</a></h3><h4 id="creating-a-standalone-migration"><a class="anchorlink" href="#creating-a-standalone-migration">2.1 Creating a Standalone Migration</a></h4><p>Migrations are stored as files in the <code>db/migrate</code> directory, one for each
migration class. The name of the file is of the form
<code>YYYYMMDDHHMMSS_create_products.rb</code>, that is to say a UTC timestamp
identifying the migration followed by an underscore followed by the name
of the migration. The name of the migration class (CamelCased version)
should match the latter part of the file name. For example
<code>20080906120000_create_products.rb</code> should define class <code>CreateProducts</code> and
<code>20080906120001_add_details_to_products.rb</code> should define
<code>AddDetailsToProducts</code>. Rails uses this timestamp to determine which migration
should be run and in what order, so if you're copying a migration from another
application or generate a file yourself, be aware of its position in the order.</p><p>Of course, calculating timestamps is no fun, so Active Record provides a
generator to handle making it for you:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails generate migration AddPartNumberToProducts
</pre>
</div>
<p>This will create an empty but appropriately named migration:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class AddPartNumberToProducts < ActiveRecord::Migration[5.0]
def change
end
end
</pre>
</div>
<p>If the migration name is of the form "AddXXXToYYY" or "RemoveXXXFromYYY" and is
followed by a list of column names and types then a migration containing the
appropriate <code>add_column</code> and <code>remove_column</code> statements will be created.</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails generate migration AddPartNumberToProducts part_number:string
</pre>
</div>
<p>will generate</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class AddPartNumberToProducts < ActiveRecord::Migration[5.0]
def change
add_column :products, :part_number, :string
end
end
</pre>
</div>
<p>If you'd like to add an index on the new column, you can do that as well:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails generate migration AddPartNumberToProducts part_number:string:index
</pre>
</div>
<p>will generate</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class AddPartNumberToProducts < ActiveRecord::Migration[5.0]
def change
add_column :products, :part_number, :string
add_index :products, :part_number
end
end
</pre>
</div>
<p>Similarly, you can generate a migration to remove a column from the command line:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails generate migration RemovePartNumberFromProducts part_number:string
</pre>
</div>
<p>generates</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class RemovePartNumberFromProducts < ActiveRecord::Migration[5.0]
def change
remove_column :products, :part_number, :string
end
end
</pre>
</div>
<p>You are not limited to one magically generated column. For example:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails generate migration AddDetailsToProducts part_number:string price:decimal
</pre>
</div>
<p>generates</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class AddDetailsToProducts < ActiveRecord::Migration[5.0]
def change
add_column :products, :part_number, :string
add_column :products, :price, :decimal
end
end
</pre>
</div>
<p>If the migration name is of the form "CreateXXX" and is
followed by a list of column names and types then a migration creating the table
XXX with the columns listed will be generated. For example:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails generate migration CreateProducts name:string part_number:string
</pre>
</div>
<p>generates</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class CreateProducts < ActiveRecord::Migration[5.0]
def change
create_table :products do |t|
t.string :name
t.string :part_number
end
end
end
</pre>
</div>
<p>As always, what has been generated for you is just a starting point. You can add
or remove from it as you see fit by editing the
<code>db/migrate/YYYYMMDDHHMMSS_add_details_to_products.rb</code> file.</p><p>Also, the generator accepts column type as <code>references</code> (also available as
<code>belongs_to</code>). For instance:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails generate migration AddUserRefToProducts user:references
</pre>
</div>
<p>generates</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class AddUserRefToProducts < ActiveRecord::Migration[5.0]
def change
add_reference :products, :user, foreign_key: true
end
end
</pre>
</div>
<p>This migration will create a <code>user_id</code> column and appropriate index.
For more <code>add_reference</code> options, visit the <a href="http://edgeapi.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_reference">API documentation</a>.</p><p>There is also a generator which will produce join tables if <code>JoinTable</code> is part of the name:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails g migration CreateJoinTableCustomerProduct customer product
</pre>
</div>
<p>will produce the following migration:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class CreateJoinTableCustomerProduct < ActiveRecord::Migration[5.0]
def change
create_join_table :customers, :products do |t|
# t.index [:customer_id, :product_id]
# t.index [:product_id, :customer_id]
end
end
end
</pre>
</div>
<h4 id="model-generators"><a class="anchorlink" href="#model-generators">2.2 Model Generators</a></h4><p>The model and scaffold generators will create migrations appropriate for adding
a new model. This migration will already contain instructions for creating the
relevant table. If you tell Rails what columns you want, then statements for
adding these columns will also be created. For example, running:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails generate model Product name:string description:text
</pre>
</div>
<p>will create a migration that looks like this</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class CreateProducts < ActiveRecord::Migration[5.0]
def change
create_table :products do |t|
t.string :name
t.text :description
t.timestamps
end
end
end
</pre>
</div>
<p>You can append as many column name/type pairs as you want.</p><h4 id="passing-modifiers"><a class="anchorlink" href="#passing-modifiers">2.3 Passing Modifiers</a></h4><p>Some commonly used <a href="#column-modifiers">type modifiers</a> can be passed directly on
the command line. They are enclosed by curly braces and follow the field type:</p><p>For instance, running:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails generate migration AddDetailsToProducts 'price:decimal{5,2}' supplier:references{polymorphic}
</pre>
</div>
<p>will produce a migration that looks like this</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class AddDetailsToProducts < ActiveRecord::Migration[5.0]
def change
add_column :products, :price, :decimal, precision: 5, scale: 2
add_reference :products, :supplier, polymorphic: true
end
end
</pre>
</div>
<div class="info"><p>Have a look at the generators help output for further details.</p></div><h3 id="writing-a-migration"><a class="anchorlink" href="#writing-a-migration">3 Writing a Migration</a></h3><p>Once you have created your migration using one of the generators it's time to
get to work!</p><h4 id="creating-a-table"><a class="anchorlink" href="#creating-a-table">3.1 Creating a Table</a></h4><p>The <code>create_table</code> method is one of the most fundamental, but most of the time,
will be generated for you from using a model or scaffold generator. A typical
use would be</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
create_table :products do |t|
t.string :name
end
</pre>
</div>
<p>which creates a <code>products</code> table with a column called <code>name</code> (and as discussed
below, an implicit <code>id</code> column).</p><p>By default, <code>create_table</code> will create a primary key called <code>id</code>. You can change
the name of the primary key with the <code>:primary_key</code> option (don't forget to
update the corresponding model) or, if you don't want a primary key at all, you
can pass the option <code>id: false</code>. If you need to pass database specific options
you can place an SQL fragment in the <code>:options</code> option. For example:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
create_table :products, options: "ENGINE=BLACKHOLE" do |t|
t.string :name, null: false
end
</pre>
</div>
<p>will append <code>ENGINE=BLACKHOLE</code> to the SQL statement used to create the table.</p><p>Also you can pass the <code>:comment</code> option with any description for the table
that will be stored in database itself and can be viewed with database administration
tools, such as MySQL Workbench or PgAdmin III. It's highly recommended to specify
comments in migrations for applications with large databases as it helps people
to understand data model and generate documentation.
Currently only the MySQL and PostgreSQL adapters support comments.</p><h4 id="creating-a-join-table"><a class="anchorlink" href="#creating-a-join-table">3.2 Creating a Join Table</a></h4><p>The migration method <code>create_join_table</code> creates an HABTM (has and belongs to
many) join table. A typical use would be:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
create_join_table :products, :categories
</pre>
</div>
<p>which creates a <code>categories_products</code> table with two columns called
<code>category_id</code> and <code>product_id</code>. These columns have the option <code>:null</code> set to
<code>false</code> by default. This can be overridden by specifying the <code>:column_options</code>
option:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
create_join_table :products, :categories, column_options: { null: true }
</pre>
</div>
<p>By default, the name of the join table comes from the union of the first two
arguments provided to create_join_table, in alphabetical order.
To customize the name of the table, provide a <code>:table_name</code> option:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
create_join_table :products, :categories, table_name: :categorization
</pre>
</div>
<p>creates a <code>categorization</code> table.</p><p><code>create_join_table</code> also accepts a block, which you can use to add indices
(which are not created by default) or additional columns:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
create_join_table :products, :categories do |t|
t.index :product_id
t.index :category_id
end
</pre>
</div>
<h4 id="changing-tables"><a class="anchorlink" href="#changing-tables">3.3 Changing Tables</a></h4><p>A close cousin of <code>create_table</code> is <code>change_table</code>, used for changing existing
tables. It is used in a similar fashion to <code>create_table</code> but the object
yielded to the block knows more tricks. For example:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
change_table :products do |t|
t.remove :description, :name
t.string :part_number
t.index :part_number
t.rename :upccode, :upc_code
end
</pre>
</div>
<p>removes the <code>description</code> and <code>name</code> columns, creates a <code>part_number</code> string
column and adds an index on it. Finally it renames the <code>upccode</code> column.</p><h4 id="changing-columns"><a class="anchorlink" href="#changing-columns">3.4 Changing Columns</a></h4><p>Like the <code>remove_column</code> and <code>add_column</code> Rails provides the <code>change_column</code>
migration method.</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
change_column :products, :part_number, :text
</pre>
</div>
<p>This changes the column <code>part_number</code> on products table to be a <code>:text</code> field.
Note that <code>change_column</code> command is irreversible.</p><p>Besides <code>change_column</code>, the <code>change_column_null</code> and <code>change_column_default</code>
methods are used specifically to change a not null constraint and default
values of a column.</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
change_column_null :products, :name, false
change_column_default :products, :approved, from: true, to: false
</pre>
</div>
<p>This sets <code>:name</code> field on products to a <code>NOT NULL</code> column and the default
value of the <code>:approved</code> field from true to false.</p><div class="note"><p>You could also write the above <code>change_column_default</code> migration as
<code>change_column_default :products, :approved, false</code>, but unlike the previous
example, this would make your migration irreversible.</p></div><h4 id="column-modifiers"><a class="anchorlink" href="#column-modifiers">3.5 Column Modifiers</a></h4><p>Column modifiers can be applied when creating or changing a column:</p>
<ul>
<li>
<code>limit</code> Sets the maximum size of the <code>string/text/binary/integer</code> fields.</li>
<li>
<code>precision</code> Defines the precision for the <code>decimal</code> fields, representing the
total number of digits in the number.</li>
<li>
<code>scale</code> Defines the scale for the <code>decimal</code> fields, representing the
number of digits after the decimal point.</li>
<li>
<code>polymorphic</code> Adds a <code>type</code> column for <code>belongs_to</code> associations.</li>
<li>
<code>null</code> Allows or disallows <code>NULL</code> values in the column.</li>
<li>
<code>default</code> Allows to set a default value on the column. Note that if you
are using a dynamic value (such as a date), the default will only be calculated
the first time (i.e. on the date the migration is applied).</li>
<li>
<code>index</code> Adds an index for the column.</li>
<li>
<code>comment</code> Adds a comment for the column.</li>
</ul>
<p>Some adapters may support additional options; see the adapter specific API docs
for further information.</p><div class="note"><p><code>null</code> and <code>default</code> cannot be specified via command line.</p></div><h4 id="foreign-keys"><a class="anchorlink" href="#foreign-keys">3.6 Foreign Keys</a></h4><p>While it's not required you might want to add foreign key constraints to
<a href="#active-record-and-referential-integrity">guarantee referential integrity</a>.</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
add_foreign_key :articles, :authors
</pre>
</div>
<p>This adds a new foreign key to the <code>author_id</code> column of the <code>articles</code>
table. The key references the <code>id</code> column of the <code>authors</code> table. If the
column names can not be derived from the table names, you can use the
<code>:column</code> and <code>:primary_key</code> options.</p><p>Rails will generate a name for every foreign key starting with
<code>fk_rails_</code> followed by 10 characters which are deterministically
generated from the <code>from_table</code> and <code>column</code>.
There is a <code>:name</code> option to specify a different name if needed.</p><div class="note"><p>Active Record only supports single column foreign keys. <code>execute</code> and
<code>structure.sql</code> are required to use composite foreign keys. See
<a href="#schema-dumping-and-you">Schema Dumping and You</a>.</p></div><div class="note"><p>The SQLite3 adapter doesn't support <code>add_foreign_key</code> since SQLite supports
only <a href="https://www.sqlite.org/lang_altertable.html">a limited subset of ALTER TABLE</a>.</p></div><p>Removing a foreign key is easy as well:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
# let Active Record figure out the column name
remove_foreign_key :accounts, :branches
# remove foreign key for a specific column
remove_foreign_key :accounts, column: :owner_id
# remove foreign key by name
remove_foreign_key :accounts, name: :special_fk_name
</pre>
</div>
<h4 id="when-helpers-aren-t-enough"><a class="anchorlink" href="#when-helpers-aren-t-enough">3.7 When Helpers aren't Enough</a></h4><p>If the helpers provided by Active Record aren't enough you can use the <code>execute</code>
method to execute arbitrary SQL:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
Product.connection.execute("UPDATE products SET price = 'free' WHERE 1=1")
</pre>
</div>
<p>For more details and examples of individual methods, check the API documentation.
In particular the documentation for
<a href="http://edgeapi.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html"><code>ActiveRecord::ConnectionAdapters::SchemaStatements</code></a>
(which provides the methods available in the <code>change</code>, <code>up</code> and <code>down</code> methods),
<a href="http://edgeapi.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/TableDefinition.html"><code>ActiveRecord::ConnectionAdapters::TableDefinition</code></a>
(which provides the methods available on the object yielded by <code>create_table</code>)
and
<a href="http://edgeapi.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/Table.html"><code>ActiveRecord::ConnectionAdapters::Table</code></a>
(which provides the methods available on the object yielded by <code>change_table</code>).</p><h4 id="using-the-change-method"><a class="anchorlink" href="#using-the-change-method">3.8 Using the <code>change</code> Method</a></h4><p>The <code>change</code> method is the primary way of writing migrations. It works for the
majority of cases, where Active Record knows how to reverse the migration
automatically. Currently, the <code>change</code> method supports only these migration
definitions:</p>
<ul>
<li>add_column</li>
<li>add_foreign_key</li>
<li>add_index</li>
<li>add_reference</li>
<li>add_timestamps</li>
<li>change_column_default (must supply a :from and :to option)</li>
<li>change_column_null</li>
<li>create_join_table</li>
<li>create_table</li>
<li>disable_extension</li>
<li>drop_join_table</li>
<li>drop_table (must supply a block)</li>
<li>enable_extension</li>
<li>remove_column (must supply a type)</li>
<li>remove_foreign_key (must supply a second table)</li>
<li>remove_index</li>
<li>remove_reference</li>
<li>remove_timestamps</li>
<li>rename_column</li>
<li>rename_index</li>
<li>rename_table</li>
</ul>
<p><code>change_table</code> is also reversible, as long as the block does not call <code>change</code>,
<code>change_default</code> or <code>remove</code>.</p><p><code>remove_column</code> is reversible if you supply the column type as the third
argument. Provide the original column options too, otherwise Rails can't
recreate the column exactly when rolling back:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
remove_column :posts, :slug, :string, null: false, default: ''
</pre>
</div>
<p>If you're going to need to use any other methods, you should use <code>reversible</code>
or write the <code>up</code> and <code>down</code> methods instead of using the <code>change</code> method.</p><h4 id="using-reversible"><a class="anchorlink" href="#using-reversible">3.9 Using <code>reversible</code></a></h4><p>Complex migrations may require processing that Active Record doesn't know how
to reverse. You can use <code>reversible</code> to specify what to do when running a
migration and what else to do when reverting it. For example:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class ExampleMigration < ActiveRecord::Migration[5.0]
def change
create_table :distributors do |t|
t.string :zipcode
end
reversible do |dir|
dir.up do
# add a CHECK constraint
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
add_column :users, :home_page_url, :string
rename_column :users, :email, :email_address
end
end
</pre>
</div>
<p>Using <code>reversible</code> will ensure that the instructions are executed in the
right order too. If the previous example migration is reverted,
the <code>down</code> block will be run after the <code>home_page_url</code> column is removed and
right before the table <code>distributors</code> is dropped.</p><p>Sometimes your migration will do something which is just plain irreversible; for
example, it might destroy some data. In such cases, you can raise
<code>ActiveRecord::IrreversibleMigration</code> in your <code>down</code> block. If someone tries
to revert your migration, an error message will be displayed saying that it
can't be done.</p><h4 id="using-the-up-down-methods"><a class="anchorlink" href="#using-the-up-down-methods">3.10 Using the <code>up</code>/<code>down</code> Methods</a></h4><p>You can also use the old style of migration using <code>up</code> and <code>down</code> methods
instead of the <code>change</code> method.
The <code>up</code> method should describe the transformation you'd like to make to your
schema, and the <code>down</code> method of your migration should revert the
transformations done by the <code>up</code> method. In other words, the database schema
should be unchanged if you do an <code>up</code> followed by a <code>down</code>. For example, if you
create a table in the <code>up</code> method, you should drop it in the <code>down</code> method. It
is wise to perform the transformations in precisely the reverse order they were
made in the <code>up</code> method. The example in the <code>reversible</code> section is equivalent to:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class ExampleMigration < ActiveRecord::Migration[5.0]
def up
create_table :distributors do |t|
t.string :zipcode
end
# add a CHECK constraint
execute <<-SQL
ALTER TABLE distributors
ADD CONSTRAINT zipchk
CHECK (char_length(zipcode) = 5);
SQL
add_column :users, :home_page_url, :string
rename_column :users, :email, :email_address
end
def down
rename_column :users, :email_address, :email
remove_column :users, :home_page_url
execute <<-SQL
ALTER TABLE distributors
DROP CONSTRAINT zipchk
SQL
drop_table :distributors
end
end
</pre>
</div>
<p>If your migration is irreversible, you should raise
<code>ActiveRecord::IrreversibleMigration</code> from your <code>down</code> method. If someone tries
to revert your migration, an error message will be displayed saying that it
can't be done.</p><h4 id="reverting-previous-migrations"><a class="anchorlink" href="#reverting-previous-migrations">3.11 Reverting Previous Migrations</a></h4><p>You can use Active Record's ability to rollback migrations using the <code>revert</code> method:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
require_relative '20121212123456_example_migration'
class FixupExampleMigration < ActiveRecord::Migration[5.0]
def change
revert ExampleMigration
create_table(:apples) do |t|
t.string :variety
end
end
end
</pre>
</div>
<p>The <code>revert</code> method also accepts a block of instructions to reverse.
This could be useful to revert selected parts of previous migrations.
For example, let's imagine that <code>ExampleMigration</code> is committed and it
is later decided it would be best to use Active Record validations,
in place of the <code>CHECK</code> constraint, to verify the zipcode.</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class DontUseConstraintForZipcodeValidationMigration < ActiveRecord::Migration[5.0]
def change
revert do
# copy-pasted code from ExampleMigration
reversible do |dir|
dir.up do
# add a CHECK constraint
execute <<-SQL
ALTER TABLE distributors
ADD CONSTRAINT zipchk
CHECK (char_length(zipcode) = 5);
SQL
end
dir.down do
execute <<-SQL
ALTER TABLE distributors
DROP CONSTRAINT zipchk
SQL
end
end
# The rest of the migration was ok
end
end
end
</pre>
</div>
<p>The same migration could also have been written without using <code>revert</code>
but this would have involved a few more steps: reversing the order
of <code>create_table</code> and <code>reversible</code>, replacing <code>create_table</code>
by <code>drop_table</code>, and finally replacing <code>up</code> by <code>down</code> and vice-versa.
This is all taken care of by <code>revert</code>.</p><div class="note"><p>If you want to add check constraints like in the examples above,
you will have to use <code>structure.sql</code> as dump method. See
<a href="#schema-dumping-and-you">Schema Dumping and You</a>.</p></div><h3 id="running-migrations"><a class="anchorlink" href="#running-migrations">4 Running Migrations</a></h3><p>Rails provides a set of rails commands to run certain sets of migrations.</p><p>The very first migration related rails command you will use will probably be
<code>rails db:migrate</code>. In its most basic form it just runs the <code>change</code> or <code>up</code>
method for all the migrations that have not yet been run. If there are
no such migrations, it exits. It will run these migrations in order based
on the date of the migration.</p><p>Note that running the <code>db:migrate</code> command also invokes the <code>db:schema:dump</code> command, which
will update your <code>db/schema.rb</code> file to match the structure of your database.</p><p>If you specify a target version, Active Record will run the required migrations
(change, up, down) until it has reached the specified version. The version
is the numerical prefix on the migration's filename. For example, to migrate
to version 20080906120000 run:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails db:migrate VERSION=20080906120000
</pre>
</div>
<p>If version 20080906120000 is greater than the current version (i.e., it is
migrating upwards), this will run the <code>change</code> (or <code>up</code>) method
on all migrations up to and
including 20080906120000, and will not execute any later migrations. If
migrating downwards, this will run the <code>down</code> method on all the migrations
down to, but not including, 20080906120000.</p><h4 id="rolling-back"><a class="anchorlink" href="#rolling-back">4.1 Rolling Back</a></h4><p>A common task is to rollback the last migration. For example, if you made a
mistake in it and wish to correct it. Rather than tracking down the version
number associated with the previous migration you can run:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails db:rollback
</pre>
</div>
<p>This will rollback the latest migration, either by reverting the <code>change</code>
method or by running the <code>down</code> method. If you need to undo
several migrations you can provide a <code>STEP</code> parameter:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails db:rollback STEP=3
</pre>
</div>
<p>will revert the last 3 migrations.</p><p>The <code>db:migrate:redo</code> command is a shortcut for doing a rollback and then migrating
back up again. As with the <code>db:rollback</code> command, you can use the <code>STEP</code> parameter
if you need to go more than one version back, for example:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails db:migrate:redo STEP=3
</pre>
</div>
<p>Neither of these rails commands do anything you could not do with <code>db:migrate</code>. They
are simply more convenient, since you do not need to explicitly specify the
version to migrate to.</p><h4 id="setup-the-database"><a class="anchorlink" href="#setup-the-database">4.2 Setup the Database</a></h4><p>The <code>rails db:setup</code> command will create the database, load the schema, and initialize
it with the seed data.</p><h4 id="resetting-the-database"><a class="anchorlink" href="#resetting-the-database">4.3 Resetting the Database</a></h4><p>The <code>rails db:reset</code> command will drop the database and set it up again. This is
functionally equivalent to <code>rails db:drop db:setup</code>.</p><div class="note"><p>This is not the same as running all the migrations. It will only use the
contents of the current <code>db/schema.rb</code> or <code>db/structure.sql</code> file. If a migration can't be rolled back,
<code>rails db:reset</code> may not help you. To find out more about dumping the schema see
<a href="#schema-dumping-and-you">Schema Dumping and You</a> section.</p></div><h4 id="running-specific-migrations"><a class="anchorlink" href="#running-specific-migrations">4.4 Running Specific Migrations</a></h4><p>If you need to run a specific migration up or down, the <code>db:migrate:up</code> and
<code>db:migrate:down</code> commands will do that. Just specify the appropriate version and
the corresponding migration will have its <code>change</code>, <code>up</code> or <code>down</code> method
invoked, for example:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails db:migrate:up VERSION=20080906120000
</pre>
</div>
<p>will run the 20080906120000 migration by running the <code>change</code> method (or the
<code>up</code> method). This command will
first check whether the migration is already performed and will do nothing if
Active Record believes that it has already been run.</p><h4 id="running-migrations-in-different-environments"><a class="anchorlink" href="#running-migrations-in-different-environments">4.5 Running Migrations in Different Environments</a></h4><p>By default running <code>rails db:migrate</code> will run in the <code>development</code> environment.
To run migrations against another environment you can specify it using the
<code>RAILS_ENV</code> environment variable while running the command. For example to run
migrations against the <code>test</code> environment you could run:</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
$ rails db:migrate RAILS_ENV=test
</pre>
</div>
<h4 id="changing-the-output-of-running-migrations"><a class="anchorlink" href="#changing-the-output-of-running-migrations">4.6 Changing the Output of Running Migrations</a></h4><p>By default migrations tell you exactly what they're doing and how long it took.
A migration creating a table and adding an index might produce output like this</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
== CreateProducts: migrating =================================================
-- create_table(:products)
-> 0.0028s
== CreateProducts: migrated (0.0028s) ========================================
</pre>
</div>
<p>Several methods are provided in migrations that allow you to control all this:</p>
<table>
<thead>
<tr>
<th>Method</th>
<th>Purpose</th>
</tr>
</thead>
<tbody>
<tr>
<td>suppress_messages</td>
<td>Takes a block as an argument and suppresses any output generated by the block.</td>
</tr>
<tr>
<td>say</td>
<td>Takes a message argument and outputs it as is. A second boolean argument can be passed to specify whether to indent or not.</td>
</tr>
<tr>
<td>say_with_time</td>
<td>Outputs text along with how long it took to run its block. If the block returns an integer it assumes it is the number of rows affected.</td>
</tr>
</tbody>
</table>
<p>For example, this migration:</p><div class="code_container">
<pre class="brush: ruby; gutter: false; toolbar: false">
class CreateProducts < ActiveRecord::Migration[5.0]
def change
suppress_messages do
create_table :products do |t|
t.string :name
t.text :description
t.timestamps
end
end
say "Created a table"
suppress_messages {add_index :products, :name}
say "and an index!", true
say_with_time 'Waiting for a while' do
sleep 10
250
end
end
end
</pre>
</div>
<p>generates the following output</p><div class="code_container">
<pre class="brush: plain; gutter: false; toolbar: false">
== CreateProducts: migrating =================================================
-- Created a table
-> and an index!
-- Waiting for a while
-> 10.0013s
-> 250 rows
== CreateProducts: migrated (10.0054s) =======================================
</pre>
</div>
<p>If you want Active Record to not output anything, then running <code>rails db:migrate
VERBOSE=false</code> will suppress all output.</p><h3 id="changing-existing-migrations"><a class="anchorlink" href="#changing-existing-migrations">5 Changing Existing Migrations</a></h3><p>Occasionally you will make a mistake when writing a migration. If you have
already run the migration, then you cannot just edit the migration and run the
migration again: Rails thinks it has already run the migration and so will do
nothing when you run <code>rails db:migrate</code>. You must rollback the migration (for
example with <code>rails db:rollback</code>), edit your migration, and then run
<code>rails db:migrate</code> to run the corrected version.</p><p>In general, editing existing migrations is not a good idea. You will be
creating extra work for yourself and your co-workers and cause major headaches
if the existing version of the migration has already been run on production
machines. Instead, you should write a new migration that performs the changes
you require. Editing a freshly generated migration that has not yet been
committed to source control (or, more generally, which has not been propagated
beyond your development machine) is relatively harmless.</p><p>The <code>revert</code> method can be helpful when writing a new migration to undo
previous migrations in whole or in part
(see <a href="#reverting-previous-migrations">Reverting Previous Migrations</a> above).</p><h3 id="schema-dumping-and-you"><a class="anchorlink" href="#schema-dumping-and-you">6 Schema Dumping and You</a></h3><h4 id="what-are-schema-files-for-questionmark"><a class="anchorlink" href="#what-are-schema-files-for-questionmark">6.1 What are Schema Files for?</a></h4><p>Migrations, mighty as they may be, are not the authoritative source for your
database schema. Your database remains the authoritative source. By default,
Rails generates <code>db/schema.rb</code> which attempts to capture the current state of
your database schema.</p><p>It tends to be faster and less error prone to create a new instance of your
application's database by loading the schema file via <code>rails db:schema:load</code>
than it is to replay the entire migration history.
<a href="#old-migrations">Old migrations</a> may fail to apply correctly if those
migrations use changing external dependencies or rely on application code which
evolves separately from your migrations.</p><p>Schema files are also useful if you want a quick look at what attributes an