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Diffstat (limited to 'guides/source/active_record_migrations.md')
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1 files changed, 117 insertions, 89 deletions
diff --git a/guides/source/active_record_migrations.md b/guides/source/active_record_migrations.md index 0e6c8c4f4a..f914122242 100644 --- a/guides/source/active_record_migrations.md +++ b/guides/source/active_record_migrations.md @@ -12,7 +12,7 @@ After reading this guide, you will know: * The generators you can use to create them. * The methods Active Record provides to manipulate your database. -* The Rake tasks that manipulate migrations and your schema. +* The bin/rails tasks that manipulate migrations and your schema. * How migrations relate to `schema.rb`. -------------------------------------------------------------------------------- @@ -35,13 +35,13 @@ history to the latest version. Active Record will also update your Here's an example of a migration: ```ruby -class CreateProducts < ActiveRecord::Migration +class CreateProducts < ActiveRecord::Migration[5.0] def change create_table :products do |t| t.string :name t.text :description - t.timestamps null: false + t.timestamps end end end @@ -72,7 +72,7 @@ If you wish for a migration to do something that Active Record doesn't know how to reverse, you can use `reversible`: ```ruby -class ChangeProductsPrice < ActiveRecord::Migration +class ChangeProductsPrice < ActiveRecord::Migration[5.0] def change reversible do |dir| change_table :products do |t| @@ -87,7 +87,7 @@ end Alternatively, you can use `up` and `down` instead of `change`: ```ruby -class ChangeProductsPrice < ActiveRecord::Migration +class ChangeProductsPrice < ActiveRecord::Migration[5.0] def up change_table :products do |t| t.change :price, :string @@ -129,7 +129,7 @@ $ bin/rails generate migration AddPartNumberToProducts This will create an empty but appropriately named migration: ```ruby -class AddPartNumberToProducts < ActiveRecord::Migration +class AddPartNumberToProducts < ActiveRecord::Migration[5.0] def change end end @@ -146,7 +146,7 @@ $ bin/rails generate migration AddPartNumberToProducts part_number:string will generate ```ruby -class AddPartNumberToProducts < ActiveRecord::Migration +class AddPartNumberToProducts < ActiveRecord::Migration[5.0] def change add_column :products, :part_number, :string end @@ -162,7 +162,7 @@ $ bin/rails generate migration AddPartNumberToProducts part_number:string:index will generate ```ruby -class AddPartNumberToProducts < ActiveRecord::Migration +class AddPartNumberToProducts < ActiveRecord::Migration[5.0] def change add_column :products, :part_number, :string add_index :products, :part_number @@ -180,7 +180,7 @@ $ bin/rails generate migration RemovePartNumberFromProducts part_number:string generates ```ruby -class RemovePartNumberFromProducts < ActiveRecord::Migration +class RemovePartNumberFromProducts < ActiveRecord::Migration[5.0] def change remove_column :products, :part_number, :string end @@ -196,7 +196,7 @@ $ bin/rails generate migration AddDetailsToProducts part_number:string price:dec generates ```ruby -class AddDetailsToProducts < ActiveRecord::Migration +class AddDetailsToProducts < ActiveRecord::Migration[5.0] def change add_column :products, :part_number, :string add_column :products, :price, :decimal @@ -215,7 +215,7 @@ $ bin/rails generate migration CreateProducts name:string part_number:string generates ```ruby -class CreateProducts < ActiveRecord::Migration +class CreateProducts < ActiveRecord::Migration[5.0] def change create_table :products do |t| t.string :name @@ -239,7 +239,7 @@ $ bin/rails generate migration AddUserRefToProducts user:references generates ```ruby -class AddUserRefToProducts < ActiveRecord::Migration +class AddUserRefToProducts < ActiveRecord::Migration[5.0] def change add_reference :products, :user, index: true, foreign_key: true end @@ -247,6 +247,7 @@ end ``` This migration will create a `user_id` column and appropriate index. +For more `add_reference` options, visit the [API documentation](http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_reference). There is also a generator which will produce join tables if `JoinTable` is part of the name: @@ -257,7 +258,7 @@ $ bin/rails g migration CreateJoinTableCustomerProduct customer product will produce the following migration: ```ruby -class CreateJoinTableCustomerProduct < ActiveRecord::Migration +class CreateJoinTableCustomerProduct < ActiveRecord::Migration[5.0] def change create_join_table :customers, :products do |t| # t.index [:customer_id, :product_id] @@ -281,13 +282,13 @@ $ bin/rails generate model Product name:string description:text will create a migration that looks like this ```ruby -class CreateProducts < ActiveRecord::Migration +class CreateProducts < ActiveRecord::Migration[5.0] def change create_table :products do |t| t.string :name t.text :description - t.timestamps null: false + t.timestamps end end end @@ -309,7 +310,7 @@ $ bin/rails generate migration AddDetailsToProducts 'price:decimal{5,2}' supplie will produce a migration that looks like this ```ruby -class AddDetailsToProducts < ActiveRecord::Migration +class AddDetailsToProducts < ActiveRecord::Migration[5.0] def change add_column :products, :price, :decimal, precision: 5, scale: 2 add_reference :products, :supplier, polymorphic: true, index: true @@ -353,12 +354,19 @@ end ``` will append `ENGINE=BLACKHOLE` to the SQL statement used to create the table -(when using MySQL, the default is `ENGINE=InnoDB`). +(when using MySQL or MariaDB, the default is `ENGINE=InnoDB`). + +Also you can pass the `:comment` 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. ### 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 @@ -367,23 +375,21 @@ 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: @@ -423,21 +429,23 @@ 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 @@ -454,8 +462,7 @@ 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. +* `comment` Adds a comment for the column. Some adapters may support additional options; see the adapter specific API docs for further information. @@ -475,7 +482,8 @@ 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 @@ -501,7 +509,7 @@ 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. @@ -521,24 +529,39 @@ 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. @@ -546,10 +569,10 @@ 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 +class ExampleMigration < ActiveRecord::Migration[5.0] def change create_table :distributors do |t| t.string :zipcode @@ -598,11 +621,11 @@ 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 -class ExampleMigration < ActiveRecord::Migration +class ExampleMigration < ActiveRecord::Migration[5.0] def up create_table :distributors do |t| t.string :zipcode @@ -643,9 +666,9 @@ 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 +class FixupExampleMigration < ActiveRecord::Migration[5.0] def change revert ExampleMigration @@ -663,7 +686,7 @@ is later decided it would be best to use Active Record validations, in place of the `CHECK` constraint, to verify the zipcode. ```ruby -class DontUseConstraintForZipcodeValidationMigration < ActiveRecord::Migration +class DontUseConstraintForZipcodeValidationMigration < ActiveRecord::Migration[5.0] def change revert do # copy-pasted code from ExampleMigration @@ -703,10 +726,10 @@ you will have to use `structure.sql` as dump method. See Running Migrations ------------------ -Rails provides a set of Rake tasks to run certain sets of migrations. +Rails provides a set of bin/rails tasks to run certain sets of migrations. -The very first migration related Rake task you will use will probably be -`rake db:migrate`. In its most basic form it just runs the `change` or `up` +The very first migration related bin/rails task you will use will probably be +`rails db:migrate`. In its most basic form it just runs the `change` or `up` 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. @@ -720,7 +743,7 @@ is the numerical prefix on the migration's filename. For example, to migrate to version 20080906120000 run: ```bash -$ bin/rake db:migrate VERSION=20080906120000 +$ bin/rails db:migrate VERSION=20080906120000 ``` If version 20080906120000 is greater than the current version (i.e., it is @@ -737,7 +760,7 @@ mistake in it and wish to correct it. Rather than tracking down the version number associated with the previous migration you can run: ```bash -$ bin/rake db:rollback +$ bin/rails db:rollback ``` This will rollback the latest migration, either by reverting the `change` @@ -745,7 +768,7 @@ method or by running the `down` method. If you need to undo several migrations you can provide a `STEP` parameter: ```bash -$ bin/rake db:rollback STEP=3 +$ bin/rails db:rollback STEP=3 ``` will revert the last 3 migrations. @@ -755,26 +778,26 @@ back up again. As with the `db:rollback` task, you can use the `STEP` parameter if you need to go more than one version back, for example: ```bash -$ bin/rake db:migrate:redo STEP=3 +$ bin/rails db:migrate:redo STEP=3 ``` -Neither of these Rake tasks do anything you could not do with `db:migrate`. They +Neither of these bin/rails tasks do anything you could not do with `db:migrate`. They are simply more convenient, since you do not need to explicitly specify the version to migrate to. ### Setup the Database -The `rake db:setup` task will create the database, load the schema and initialize +The `rails db:setup` task will create the database, load the schema and initialize it with the seed data. ### Resetting the Database -The `rake db:reset` task will drop the database and set it up again. This is -functionally equivalent to `rake db:drop db:setup`. +The `rails db:reset` task will drop the database and set it up again. This is +functionally equivalent to `rails 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, -`rake db:reset` may not help you. To find out more about dumping the schema see +contents of the current `db/schema.rb` or `db/structure.sql` file. If a migration can't be rolled back, +`rails db:reset` may not help you. To find out more about dumping the schema see [Schema Dumping and You](#schema-dumping-and-you) section. ### Running Specific Migrations @@ -785,7 +808,7 @@ the corresponding migration will have its `change`, `up` or `down` method invoked, for example: ```bash -$ bin/rake db:migrate:up VERSION=20080906120000 +$ bin/rails db:migrate:up VERSION=20080906120000 ``` will run the 20080906120000 migration by running the `change` method (or the @@ -795,13 +818,13 @@ Active Record believes that it has already been run. ### Running Migrations in Different Environments -By default running `rake db:migrate` will run in the `development` environment. +By default running `bin/rails db:migrate` will run in the `development` environment. To run migrations against another environment you can specify it using the `RAILS_ENV` environment variable while running the command. For example to run migrations against the `test` environment you could run: ```bash -$ bin/rake db:migrate RAILS_ENV=test +$ bin/rails db:migrate RAILS_ENV=test ``` ### Changing the Output of Running Migrations @@ -827,13 +850,13 @@ Several methods are provided in migrations that allow you to control all this: For example, this migration: ```ruby -class CreateProducts < ActiveRecord::Migration +class CreateProducts < ActiveRecord::Migration[5.0] def change suppress_messages do create_table :products do |t| t.string :name t.text :description - t.timestamps null: false + t.timestamps end end @@ -862,18 +885,18 @@ generates the following output == CreateProducts: migrated (10.0054s) ======================================= ``` -If you want Active Record to not output anything, then running `rake db:migrate +If you want Active Record to not output anything, then running `rails db:migrate VERBOSE=false` will suppress all output. Changing Existing Migrations ---------------------------- 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 +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 `rake db:migrate`. You must rollback the migration (for -example with `rake db:rollback`), edit your migration and then run -`rake db:migrate` to run the corrected version. +nothing when you run `rails db:migrate`. You must rollback the migration (for +example with `bin/rails db:rollback`), edit your migration and then run +`rails db:migrate` to run the corrected version. 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 @@ -919,7 +942,7 @@ There are two ways to dump the schema. This is set in `config/application.rb` by the `config.active_record.schema_format` setting, which may be either `:sql` or `:ruby`. -If `:ruby` is selected then the schema is stored in `db/schema.rb`. If you look +If `:ruby` is selected, then the schema is stored in `db/schema.rb`. If you look at this file you'll find that it looks an awful lot like one very big migration: @@ -955,8 +978,8 @@ 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` -Rake task) into `db/structure.sql`. For example, for PostgreSQL, the `pg_dump` -utility is used. For MySQL, this file will contain the output of +rails task) into `db/structure.sql`. For example, for PostgreSQL, the `pg_dump` +utility is used. For MySQL and MariaDB, this file will contain the output of `SHOW CREATE TABLE` for the various tables. Loading these schemas is simply a question of executing the SQL statements they @@ -995,10 +1018,13 @@ 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 +class AddInitialProducts < ActiveRecord::Migration[5.0] def up 5.times do |i| Product.create(name: "Product ##{i}", description: "A product.") @@ -1011,9 +1037,11 @@ 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 `rails db:seed`: ```ruby 5.times do |i| |