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Diffstat (limited to 'guides/source/active_record_migrations.md')
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1 files changed, 80 insertions, 50 deletions
diff --git a/guides/source/active_record_migrations.md b/guides/source/active_record_migrations.md index 229c6ee458..67881e6087 100644 --- a/guides/source/active_record_migrations.md +++ b/guides/source/active_record_migrations.md @@ -1,3 +1,5 @@ +**DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON http://guides.rubyonrails.org.** + Active Record Migrations ======================== @@ -39,7 +41,7 @@ class CreateProducts < ActiveRecord::Migration t.string :name t.text :description - t.timestamps + t.timestamps null: false end end end @@ -239,7 +241,7 @@ generates ```ruby class AddUserRefToProducts < ActiveRecord::Migration def change - add_reference :products, :user, index: true + add_reference :products, :user, index: true, foreign_key: true end end ``` @@ -285,7 +287,7 @@ class CreateProducts < ActiveRecord::Migration t.string :name t.text :description - t.timestamps + t.timestamps null: false end end end @@ -355,8 +357,8 @@ will append `ENGINE=BLACKHOLE` to the SQL statement used to create the table ### Creating a Join Table -Migration method `create_join_table` creates a HABTM join table. A typical use -would be: +The migration method `create_join_table` creates an HABTM (has and belongs to +many) join table. A typical use would be: ```ruby create_join_table :products, :categories @@ -365,23 +367,21 @@ create_join_table :products, :categories which creates a `categories_products` table with two columns called `category_id` and `product_id`. These columns have the option `:null` set to `false` by default. This can be overridden by specifying the `:column_options` -option. +option: ```ruby -create_join_table :products, :categories, column_options: {null: true} +create_join_table :products, :categories, column_options: { null: true } ``` -will create the `product_id` and `category_id` with the `:null` option as -`true`. - -You can pass the option `:table_name` when you want to customize the table -name. For example: +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 `:table_name` option: ```ruby create_join_table :products, :categories, table_name: :categorization ``` -will create a `categorization` table. +creates a `categorization` table. `create_join_table` also accepts a block, which you can use to add indices (which are not created by default) or additional columns: @@ -421,21 +421,23 @@ change_column :products, :part_number, :text ``` This changes the column `part_number` on products table to be a `:text` field. +Note that `change_column` command is irreversible. Besides `change_column`, the `change_column_null` and `change_column_default` -methods are used specifically to change the null and default values of a -column. +methods are used specifically to change a not null constraint and default +values of a column. ```ruby change_column_null :products, :name, false -change_column_default :products, :approved, false +change_column_default :products, :approved, from: true, to: false ``` This sets `:name` field on products to a `NOT NULL` column and the default -value of the `:approved` field to false. +value of the `:approved` field from true to false. -TIP: Unlike `change_column` (and `change_column_default`), `change_column_null` -is reversible. +Note: You could also write the above `change_column_default` migration as +`change_column_default :products, :approved, false`, but unlike the previous +example, this would make your migration irreversible. ### Column Modifiers @@ -452,6 +454,8 @@ number of digits after the decimal point. 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). * `index` Adds an index for the column. +* `required` Adds `required: true` for `belongs_to` associations and +`null: false` to the column in the migration. Some adapters may support additional options; see the adapter specific API docs for further information. @@ -466,16 +470,18 @@ add_foreign_key :articles, :authors ``` This adds a new foreign key to the `author_id` column of the `articles` -table. The key references the `id` column of the `articles` table. If the +table. The key references the `id` column of the `authors` table. If the column names can not be derived from the table names, you can use the `:column` and `:primary_key` options. Rails will generate a name for every foreign key starting with -`fk_rails_` followed by 10 random characters. +`fk_rails_` followed by 10 characters which are deterministically +generated from the `from_table` and `column`. There is a `:name` option to specify a different name if needed. NOTE: Active Record only supports single column foreign keys. `execute` and -`structure.sql` are required to use composite foreign keys. +`structure.sql` are required to use composite foreign keys. See +[Schema Dumping and You](#schema-dumping-and-you). Removing a foreign key is easy as well: @@ -496,7 +502,7 @@ If the helpers provided by Active Record aren't enough you can use the `execute` method to execute arbitrary SQL: ```ruby -Product.connection.execute('UPDATE `products` SET `price`=`free` WHERE 1') +Product.connection.execute("UPDATE products SET price = 'free' WHERE 1=1") ``` For more details and examples of individual methods, check the API documentation. @@ -516,24 +522,39 @@ majority of cases, where Active Record knows how to reverse the migration automatically. Currently, the `change` method supports only these migration definitions: -* `add_column` -* `add_index` -* `add_reference` -* `add_timestamps` -* `add_foreign_key` -* `create_table` -* `create_join_table` -* `drop_table` (must supply a block) -* `drop_join_table` (must supply a block) -* `remove_timestamps` -* `rename_column` -* `rename_index` -* `remove_reference` -* `rename_table` +* add_column +* add_foreign_key +* add_index +* add_reference +* add_timestamps +* change_column_default (must supply a :from and :to option) +* change_column_null +* create_join_table +* create_table +* disable_extension +* drop_join_table +* drop_table (must supply a block) +* enable_extension +* remove_column (must supply a type) +* remove_foreign_key (must supply a second table) +* remove_index +* remove_reference +* remove_timestamps +* rename_column +* rename_index +* rename_table `change_table` is also reversible, as long as the block does not call `change`, `change_default` or `remove`. +`remove_column` 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: + +```ruby +remove_column :posts, :slug, :string, null: false, default: '', index: true +``` + If you're going to need to use any other methods, you should use `reversible` or write the `up` and `down` methods instead of using the `change` method. @@ -541,7 +562,7 @@ or write the `up` and `down` methods instead of using the `change` method. Complex migrations may require processing that Active Record doesn't know how to reverse. You can use `reversible` to specify what to do when running a -migration what else to do when reverting it. For example: +migration and what else to do when reverting it. For example: ```ruby class ExampleMigration < ActiveRecord::Migration @@ -593,7 +614,7 @@ schema, and the `down` method of your migration should revert the transformations done by the `up` method. In other words, the database schema should be unchanged if you do an `up` followed by a `down`. For example, if you create a table in the `up` method, you should drop it in the `down` method. It -is wise to reverse the transformations in precisely the reverse order they were +is wise to perform the transformations in precisely the reverse order they were made in the `up` method. The example in the `reversible` section is equivalent to: ```ruby @@ -638,7 +659,7 @@ can't be done. You can use Active Record's ability to rollback migrations using the `revert` method: ```ruby -require_relative '2012121212_example_migration' +require_relative '20121212123456_example_migration' class FixupExampleMigration < ActiveRecord::Migration def change @@ -691,6 +712,10 @@ of `create_table` and `reversible`, replacing `create_table` by `drop_table`, and finally replacing `up` by `down` and vice-versa. This is all taken care of by `revert`. +NOTE: If you want to add check constraints like in the examples above, +you will have to use `structure.sql` as dump method. See +[Schema Dumping and You](#schema-dumping-and-you). + Running Migrations ------------------ @@ -764,7 +789,7 @@ The `rake db:reset` task will drop the database and set it up again. This is functionally equivalent to `rake db:drop db:setup`. NOTE: This is not the same as running all the migrations. It will only use the -contents of the current `schema.rb` file. If a migration can't be rolled back, +contents of the current `db/schema.rb` or `db/structure.sql` file. If a migration can't be rolled back, `rake db:reset` may not help you. To find out more about dumping the schema see [Schema Dumping and You](#schema-dumping-and-you) section. @@ -824,7 +849,7 @@ class CreateProducts < ActiveRecord::Migration create_table :products do |t| t.string :name t.text :description - t.timestamps + t.timestamps null: false end end @@ -939,10 +964,10 @@ that Active Record supports. This could be very useful if you were to distribute an application that is able to run against multiple databases. There is however a trade-off: `db/schema.rb` cannot express database specific -items such as triggers, or stored procedures. While in a migration you can -execute custom SQL statements, the schema dumper cannot reconstitute those -statements from the database. If you are using features like this, then you -should set the schema format to `:sql`. +items such as triggers, stored procedures or check constraints. While in a +migration you can execute custom SQL statements, the schema dumper cannot +reconstitute those statements from the database. If you are using features like +this, then you should set the schema format to `:sql`. Instead of using Active Record's schema dumper, the database's structure will be dumped using a tool specific to the database (via the `db:structure:dump` @@ -986,7 +1011,10 @@ such features, the `execute` method can be used to execute arbitrary SQL. Migrations and Seed Data ------------------------ -Some people use migrations to add data to the database: +The main purpose of Rails' migration feature is to issue commands that modify the +schema using a consistent process. Migrations can also be used +to add or modify data. This is useful in an existing database that can't be destroyed +and recreated, such as a production database. ```ruby class AddInitialProducts < ActiveRecord::Migration @@ -1002,9 +1030,11 @@ class AddInitialProducts < ActiveRecord::Migration end ``` -However, Rails has a 'seeds' feature that should be used for seeding a database -with initial data. It's a really simple feature: just fill up `db/seeds.rb` -with some Ruby code, and run `rake db:seed`: +To add initial data after a database is created, Rails has a built-in +'seeds' feature that makes the process quick and easy. This is especially +useful when reloading the database frequently in development and test environments. +It's easy to get started with this feature: just fill up `db/seeds.rb` with some +Ruby code, and run `rake db:seed`: ```ruby 5.times do |i| |