Chapters

1. Migration Overview
2. Creating a Migration
   • Creating a Standalone Migration
   • Model Generators
   • Passing Modifiers
3. Writing a Migration
   • Creating a Table
   • Creating a Join Table
   • Changing Tables
   • Changing Columns
   • Column Modifiers
   • Foreign Keys
   • When Helpers aren't Enough
   • Using the change Method
   • Using reversible
   • Using the up/down Methods
   • Reverting Previous Migrations
4. Running Migrations
   • Rolling Back
   • Setup the Database
   • Resetting the Database
   • Running Specific Migrations
   • Running Migrations in Different Environments
   • Changing the Output of Running Migrations
5. Changing Existing Migrations
6. Schema Dumping and You
   • What are Schema Files for?
   • Types of Schema Dumps
   • Schema Dumps and Source Control
7. Active Record and Referential Integrity
8. Migrations and Seed Data

1 Migration Overview

Migrations are a convenient way to alter your database schema over time 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.

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 db/schema.rb file to match the up-to-date structure of your database.

Here's an example of a migration:

class CreateProducts < ActiveRecord::Migration
  def change
    create_table :products do |t|
      t.string :name
      t.text :description

      t.timestamps null: false
    end
  end
end

This migration adds a table called products with a string column called name and a text column called description. A primary key column called id will also be added implicitly, as it's the default primary key for all Active Record models. The timestamps macro adds two columns, created_at and updated_at. These special columns are automatically managed by Active Record if they exist.

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.

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.

If you wish for a migration to do something that Active Record doesn't know how to reverse, you can use reversible:

class ChangeProductsPrice < ActiveRecord::Migration
  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

Alternatively, you can use up and down instead of change:

class ChangeProductsPrice < ActiveRecord::Migration
  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

2 Creating a Migration

2.1 Creating a Standalone Migration

Migrations are stored as files in the db/migrate directory, one for each migration class. The name of the file is of the form YYYYMMDDHHMMSS_create_products.rb, 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 20080906120000_create_products.rb should define class CreateProducts and 20080906120001_add_details_to_products.rb should define AddDetailsToProducts. 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.

Of course, calculating timestamps is no fun, so Active Record provides a generator to handle making it for you:

$ bin/rails generate migration AddPartNumberToProducts

This will create an empty but appropriately named migration:

class AddPartNumberToProducts < ActiveRecord::Migration
  def change
  end
end

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 add_column and remove_column statements will be created.

$ bin/rails generate migration AddPartNumberToProducts part_number:string

will generate

class AddPartNumberToProducts < ActiveRecord::Migration
  def change
    add_column :products, :part_number, :string
  end
end

If you'd like to add an index on the new column, you can do that as well:

$ bin/rails generate migration AddPartNumberToProducts part_number:string:index

will generate

class AddPartNumberToProducts < ActiveRecord::Migration
  def change
    add_column :products, :part_number, :string
    add_index :products, :part_number
  end
end

Similarly, you can generate a migration to remove a column from the command line:

$ bin/rails generate migration RemovePartNumberFromProducts part_number:string

generates

class RemovePartNumberFromProducts < ActiveRecord::Migration
  def change
    remove_column :products, :part_number, :string
  end
end

You are not limited to one magically generated column. For example:

$ bin/rails generate migration AddDetailsToProducts part_number:string price:decimal

generates

class AddDetailsToProducts < ActiveRecord::Migration
  def change
    add_column :products, :part_number, :string
    add_column :products, :price, :decimal
  end
end

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:

$ bin/rails generate migration CreateProducts name:string part_number:string

generates

class CreateProducts < ActiveRecord::Migration
  def change
    create_table :products do |t|
      t.string :name
      t.string :part_number
    end
  end
end

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 db/migrate/YYYYMMDDHHMMSS_add_details_to_products.rb file.

Also, the generator accepts column type as references(also available as belongs_to). For instance:

$ bin/rails generate migration AddUserRefToProducts user:references

generates

class AddUserRefToProducts < ActiveRecord::Migration
  def change
    add_reference :products, :user, index: true
  end
end

This migration will create a user_id column and appropriate index.

There is also a generator which will produce join tables if JoinTable is part of the name:

$ bin/rails g migration CreateJoinTableCustomerProduct customer product

will produce the following migration:

class CreateJoinTableCustomerProduct < ActiveRecord::Migration
  def change
    create_join_table :customers, :products do |t|
      # t.index [:customer_id, :product_id]
      # t.index [:product_id, :customer_id]
    end
  end
end

2.2 Model Generators

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:

$ bin/rails generate model Product name:string description:text

will create a migration that looks like this

class CreateProducts < ActiveRecord::Migration
  def change
    create_table :products do |t|
      t.string :name
      t.text :description

      t.timestamps null: false
    end
  end
end

You can append as many column name/type pairs as you want.

2.3 Passing Modifiers

Some commonly used type modifiers can be passed directly on the command line. They are enclosed by curly braces and follow the field type:

For instance, running:

$ bin/rails generate migration AddDetailsToProducts 'price:decimal{5,2}' supplier:references{polymorphic}

will produce a migration that looks like this

class AddDetailsToProducts < ActiveRecord::Migration
  def change
    add_column :products, :price, :decimal, precision: 5, scale: 2
    add_reference :products, :supplier, polymorphic: true, index: true
  end
end

3 Writing a Migration

Once you have created your migration using one of the generators it's time to get to work!

3.1 Creating a Table

The create_table 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

create_table :products do |t|
  t.string :name
end

which creates a products table with a column called name (and as discussed below, an implicit id column).

By default, create_table will create a primary key called id. You can change the name of the primary key with the :primary_key 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 id: false. If you need to pass database specific options you can place an SQL fragment in the :options option. For example:

create_table :products, options: "ENGINE=BLACKHOLE" do |t|
  t.string :name, null: false
end

will append ENGINE=BLACKHOLE to the SQL statement used to create the table (when using MySQL, the default is ENGINE=InnoDB).

3.2 Creating a Join Table

Migration method create_join_table creates a HABTM join table. A typical use would be:

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.

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:

create_join_table :products, :categories, table_name: :categorization

will create 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:

create_join_table :products, :categories do |t|
  t.index :product_id
  t.index :category_id
end

3.3 Changing Tables

A close cousin of create_table is change_table, used for changing existing tables. It is used in a similar fashion to create_table but the object yielded to the block knows more tricks. For example:

change_table :products do |t|
  t.remove :description, :name
  t.string :part_number
  t.index :part_number
  t.rename :upccode, :upc_code
end

removes the description and name columns, creates a part_number string column and adds an index on it. Finally it renames the upccode column.

3.4 Changing Columns

Like the remove_column and add_column Rails provides the change_column migration method.

change_column :products, :part_number, :text

This changes the column part_number on products table to be a :text field.

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.

change_column_null :products, :name, false
change_column_default :products, :approved, false

This sets :name field on products to a NOT NULL column and the default value of the :approved field to false.

3.5 Column Modifiers

Column modifiers can be applied when creating or changing a column:

• limit Sets the maximum size of the string/text/binary/integer fields.
• precision Defines the precision for the decimal fields, representing the total number of digits in the number.
• scale Defines the scale for the decimal fields, representing the number of digits after the decimal point.
• polymorphic Adds a type column for belongs_to associations.
• null Allows or disallows NULL values in the column.
• default 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).
• 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.

3.6 Foreign Keys

While it's not required you might want to add foreign key constraints to guarantee referential integrity.

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 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. There is a :name option to specify a different name if needed.

Removing a foreign key is easy as well:

# 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

3.7 When Helpers aren't Enough

If the helpers provided by Active Record aren't enough you can use the execute method to execute arbitrary SQL:

Product.connection.execute('UPDATE `products` SET `price`=`free` WHERE 1')

For more details and examples of individual methods, check the API documentation. In particular the documentation for ActiveRecord::ConnectionAdapters::SchemaStatements (which provides the methods available in the change, up and down methods), ActiveRecord::ConnectionAdapters::TableDefinition (which provides the methods available on the object yielded by create_table) and ActiveRecord::ConnectionAdapters::Table (which provides the methods available on the object yielded by change_table).

3.8 Using the change Method

The change 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 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

change_table is also reversible, as long as the block does not call change, change_default or remove.

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.

3.9 Using reversible

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:

class ExampleMigration < ActiveRecord::Migration
  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

Using reversible will ensure that the instructions are executed in the right order too. If the previous example migration is reverted, the down block will be run after the home_page_url column is removed and right before the table distributors is dropped.

Sometimes your migration will do something which is just plain irreversible; for example, it might destroy some data. In such cases, you can raise ActiveRecord::IrreversibleMigration in your down block. If someone tries to revert your migration, an error message will be displayed saying that it can't be done.

3.10 Using the up/down Methods

You can also use the old style of migration using up and down methods instead of the change method. The up method should describe the transformation you'd like to make to your 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 made in the up method. The example in the reversible section is equivalent to:

class ExampleMigration < ActiveRecord::Migration
  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

If your migration is irreversible, you should raise ActiveRecord::IrreversibleMigration from your down method. If someone tries to revert your migration, an error message will be displayed saying that it can't be done.

3.11 Reverting Previous Migrations

You can use Active Record's ability to rollback migrations using the revert method:

require_relative '2012121212_example_migration'

class FixupExampleMigration < ActiveRecord::Migration
  def change
    revert ExampleMigration

    create_table(:apples) do |t|
      t.string :variety
    end
  end
end

The revert 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 ExampleMigration is committed and it is later decided it would be best to use Active Record validations, in place of the CHECK constraint, to verify the zipcode.

class DontUseConstraintForZipcodeValidationMigration < ActiveRecord::Migration
  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

The same migration could also have been written without using revert but this would have involved a few more steps: reversing the order 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.

4 Running Migrations

Rails provides a set of Rake 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 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.

Note that running the db:migrate task also invokes the db:schema:dump task, which will update your db/schema.rb file to match the structure of your database.

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:

$ bin/rake db:migrate VERSION=20080906120000

If version 20080906120000 is greater than the current version (i.e., it is migrating upwards), this will run the change (or up) method on all migrations up to and including 20080906120000, and will not execute any later migrations. If migrating downwards, this will run the down method on all the migrations down to, but not including, 20080906120000.

4.1 Rolling Back

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:

$ bin/rake db:rollback

This will rollback the latest migration, either by reverting the change method or by running the down method. If you need to undo several migrations you can provide a STEP parameter:

$ bin/rake db:rollback STEP=3

will revert the last 3 migrations.

The db:migrate:redo task is a shortcut for doing a rollback and then migrating 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:

$ bin/rake db:migrate:redo STEP=3

Neither of these Rake 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.

4.2 Setup the Database

The rake db:setup task will create the database, load the schema and initialize it with the seed data.

4.3 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.

4.4 Running Specific Migrations

If you need to run a specific migration up or down, the db:migrate:up and db:migrate:down tasks will do that. Just specify the appropriate version and the corresponding migration will have its change, up or down method invoked, for example:

$ bin/rake db:migrate:up VERSION=20080906120000

will run the 20080906120000 migration by running the change method (or the up method). This task will first check whether the migration is already performed and will do nothing if Active Record believes that it has already been run.

4.5 Running Migrations in Different Environments

By default running rake 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:

$ bin/rake db:migrate RAILS_ENV=test

4.6 Changing the Output of Running Migrations

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

==  CreateProducts: migrating =================================================
-- create_table(:products)
   -> 0.0028s
==  CreateProducts: migrated (0.0028s) ========================================

Several methods are provided in migrations that allow you to control all this:

For example, this migration:

class CreateProducts < ActiveRecord::Migration
  def change
    suppress_messages do
      create_table :products do |t|
        t.string :name
        t.text :description
        t.timestamps null: false
      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

generates the following output

==  CreateProducts: migrating =================================================
-- Created a table
   -> and an index!
-- Waiting for a while
   -> 10.0013s
   -> 250 rows
==  CreateProducts: migrated (10.0054s) =======================================

If you want Active Record to not output anything, then running rake db:migrate VERBOSE=false will suppress all output.

5 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 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.

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.

The revert method can be helpful when writing a new migration to undo previous migrations in whole or in part (see Reverting Previous Migrations above).

6 Schema Dumping and You

6.1 What are Schema Files for?

Migrations, mighty as they may be, are not the authoritative source for your database schema. That role falls to either db/schema.rb or an SQL file which Active Record generates by examining the database. They are not designed to be edited, they just represent the current state of the database.

There is no need (and it is error prone) to deploy a new instance of an app by replaying the entire migration history. It is much simpler and faster to just load into the database a description of the current schema.

For example, this is how the test database is created: the current development database is dumped (either to db/schema.rb or db/structure.sql) and then loaded into the test database.

Schema files are also useful if you want a quick look at what attributes an Active Record object has. This information is not in the model's code and is frequently spread across several migrations, but the information is nicely summed up in the schema file. The annotate_models gem automatically adds and updates comments at the top of each model summarizing the schema if you desire that functionality.

6.2 Types of Schema Dumps

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 at this file you'll find that it looks an awful lot like one very big migration:

ActiveRecord::Schema.define(version: 20080906171750) do
  create_table "authors", force: true do |t|
    t.string   "name"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "products", force: true do |t|
    t.string   "name"
    t.text "description"
    t.datetime "created_at"
    t.datetime "updated_at"
    t.string "part_number"
  end
end

In many ways this is exactly what it is. This file is created by inspecting the database and expressing its structure using create_table, add_index, and so on. Because this is database-independent, it could be loaded into any database 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.

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 SHOW CREATE TABLE for the various tables.

Loading these schemas is simply a question of executing the SQL statements they contain. By definition, this will create a perfect copy of the database's structure. Using the :sql schema format will, however, prevent loading the schema into a RDBMS other than the one used to create it.

6.3 Schema Dumps and Source Control

Because schema dumps are the authoritative source for your database schema, it is strongly recommended that you check them into source control.

db/schema.rb contains the current version number of the database. This ensures conflicts are going to happen in the case of a merge where both branches touched the schema. When that happens, solve conflicts manually, keeping the highest version number of the two.

7 Active Record and Referential Integrity

The Active Record way claims that intelligence belongs in your models, not in the database. As such, features such as triggers or constraints, which push some of that intelligence back into the database, are not heavily used.

Validations such as validates :foreign_key, uniqueness: true are one way in which models can enforce data integrity. The :dependent option on associations allows models to automatically destroy child objects when the parent is destroyed. Like anything which operates at the application level, these cannot guarantee referential integrity and so some people augment them with foreign key constraints in the database.

Although Active Record does not provide all the tools for working directly with such features, the execute method can be used to execute arbitrary SQL.

8 Migrations and Seed Data

Some people use migrations to add data to the database:

class AddInitialProducts < ActiveRecord::Migration
  def up
    5.times do |i|
      Product.create(name: "Product ##{i}", description: "A product.")
    end
  end

  def down
    Product.delete_all
  end
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:

5.times do |i|
  Product.create(name: "Product ##{i}", description: "A product.")
end

This is generally a much cleaner way to set up the database of a blank application.

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