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Migrations

Migrations are a convenient way for you to alter your database in a structured and organized manner. You could edit fragments of SQL by hand but you would then be responsible for telling other developers that they need to go and run them. You'd also have to keep track of which changes need to be run against the production machines next time you deploy.

Active Record tracks which migrations have already been run so all you have to do is update your source and run rake db:migrate. Active Record will work out which migrations should be run. Active Record will also update your db/schema.rb file to match the up-to-date structure of your database.

Migrations also allow you to describe these transformations using Ruby. The great thing about this is that (like most of Active Record's functionality) it is database independent: you don't need to worry about the precise syntax of CREATE TABLE any more than you worry about variations on SELECT * (you can drop down to raw SQL for database specific features). For example, you could use SQLite3 in development, but MySQL in production.

In this guide, you'll learn all about migrations including:

  • The generators you can use to create them
  • The methods Active Record provides to manipulate your database
  • The Rake tasks that manipulate them
  • How they relate to schema.rb

Anatomy of a Migration

Before we dive into the details of a migration, here are a few examples of the sorts of things you can do:

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

      t.timestamps
    end
  end

  def down
    drop_table :products
  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, however since this is the default we do not need to explicitly specify it. The timestamp columns created_at and updated_at which Active Record populates automatically will also be added. Reversing this migration is as simple as dropping the table.

Migrations are not limited to changing the schema. You can also use them to fix bad data in the database or populate new fields:

class AddReceiveNewsletterToUsers < ActiveRecord::Migration
  def up
    change_table :users do |t|
      t.boolean :receive_newsletter, :default => false
    end
    User.update_all :receive_newsletter => true
  end

  def down
    remove_column :users, :receive_newsletter
  end
end

NOTE: Some caveats apply to using models in your migrations.

This migration adds a receive_newsletter column to the users table. We want it to default to false for new users, but existing users are considered to have already opted in, so we use the User model to set the flag to true for existing users.

Using the change method

Rails 3.1 and up makes migrations smarter by providing a change method. This method is preferred for writing constructive migrations (adding columns or tables). The migration knows how to migrate your database and reverse it when the migration is rolled back without the need to write a separate down method.

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

      t.timestamps
    end
  end
end

Migrations are Classes

A migration is a subclass of ActiveRecord::Migration that implements two methods: up (perform the required transformations) and down (revert them).

Active Record provides methods that perform common data definition tasks in a database independent way (you'll read about them in detail later):

  • add_column
  • add_reference
  • add_index
  • change_column
  • change_table
  • create_table
  • create_join_table
  • drop_table
  • remove_column
  • remove_index
  • rename_column
  • remove_reference

If you need to perform tasks specific to your database (e.g., create a foreign key constraint) then the execute method allows you to execute arbitrary SQL. A migration is just a regular Ruby class so you're not limited to these functions. For example, after adding a column you could write code to set the value of that column for existing records (if necessary using your models).

On databases that support transactions with statements that change the schema (such as PostgreSQL or SQLite3), migrations are wrapped in a transaction. If the database does not support this (for example MySQL) 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.

What's in a Name

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. If you do feel the need to change the file name then you have to update the name of the class inside or Rails will complain about a missing class.

Internally Rails only uses the migration's number (the timestamp) to identify them. Prior to Rails 2.1 the migration number started at 1 and was incremented each time a migration was generated. With multiple developers it was easy for these to clash requiring you to rollback migrations and renumber them. With Rails 2.1+ this is largely avoided by using the creation time of the migration to identify them. You can revert to the old numbering scheme by adding the following line to config/application.rb.

config.active_record.timestamped_migrations = false

The combination of timestamps and recording which migrations have been run allows Rails to handle common situations that occur with multiple developers.

For example, Alice adds migrations 20080906120000 and 20080906123000 and Bob adds 20080906124500 and runs it. Alice finishes her changes and checks in her migrations and Bob pulls down the latest changes. When Bob runs rake db:migrate, Rails knows that it has not run Alice's two migrations so it executes the up method for each migration.

Of course this is no substitution for communication within the team. For example, if Alice's migration removed a table that Bob's migration assumed to exist, then trouble would certainly strike.

Changing 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.

Supported Types

Active Record supports the following database column types:

  • :binary
  • :boolean
  • :date
  • :datetime
  • :decimal
  • :float
  • :integer
  • :primary_key
  • :string
  • :text
  • :time
  • :timestamp

These will be mapped onto an appropriate underlying database type. For example, with MySQL the type :string is mapped to VARCHAR(255). You can create columns of types not supported by Active Record when using the non-sexy syntax such as

create_table :products do |t|
  t.column :name, 'polygon', :null => false
end

This may however hinder portability to other databases.

Creating a Migration

Creating a Model

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

$ rails generate model Product name:string description:text

TIP: All lines starting with a dollar sign $ are intended to be run on the command line.

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
    end
  end
end

You can append as many column name/type pairs as you want. By default, the generated migration will include t.timestamps (which creates the updated_at and created_at columns that are automatically populated by Active Record).

Creating a Standalone Migration

If you are creating migrations for other purposes (e.g., to add a column to an existing table) then you can also use the migration generator:

$ 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.

$ rails generate migration AddPartNumberToProducts part_number:string

will generate

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

Similarly,

$ rails generate migration RemovePartNumberFromProducts part_number:string

generates

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

  def down
    add_column :products, :part_number, :string
  end
end

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

$ 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

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.

NOTE: The generated migration file for destructive migrations will still be old-style using the up and down methods. This is because Rails needs to know the original data types defined when you made the original changes.

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

$ 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.

Supported Type Modifiers

You can also specify some options just after the field type between curly braces. You can use the following modifiers:

  • limit Sets the maximum size of the string/text/binary/integer fields
  • precision Defines the precision for the decimal fields
  • scale Defines the scale for the decimal fields
  • polymorphic Adds a type column for belongs_to associations

For instance, running

$ 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, :precision => 5, :scale => 2
    add_reference :products, :user, :polymorphic => true, :index => true
  end
end

Writing a Migration

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

Creating a Table

Migration method create_table will be one of your workhorses. 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).

The object yielded to the block allows you to create columns on the table. There are two ways of doing it. The first (traditional) form looks like

create_table :products do |t|
  t.column :name, :string, :null => false
end

The second form, the so called "sexy" migration, drops the somewhat redundant column method. Instead, the string, integer, etc. methods create a column of that type. Subsequent parameters are the same.

create_table :products do |t|
  t.string :name, :null => false
end

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 (for example for a HABTM join table), 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).

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.

You can pass the option :table_name with you want to customize the table name. For example,

create_join_table :products, :categories, :table_name => :categorization

will create a categorization table.

By default, create_join_table will create two columns with no options, but you can specify these options using the :column_options option. For example,

create_join_table :products, :categories, :column_options => {:null => true}

will create the product_id and category_id with the :null option as true.

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.

Special Helpers

Active Record provides some shortcuts for common functionality. It is for example very common to add both the created_at and updated_at columns and so there is a method that does exactly that:

create_table :products do |t|
  t.timestamps
end

will create a new products table with those two columns (plus the id column) whereas

change_table :products do |t|
  t.timestamps
end

adds those columns to an existing table.

Another helper is called references (also available as belongs_to). In its simplest form it just adds some readability.

create_table :products do |t|
  t.references :category
end

will create a category_id column of the appropriate type. Note that you pass the model name, not the column name. Active Record adds the _id for you. If you have polymorphic belongs_to associations then references will add both of the columns required:

create_table :products do |t|
  t.references :attachment, :polymorphic => {:default => 'Photo'}
end

will add an attachment_id column and a string attachment_type column with a default value of 'Photo'. references also allows you to define an index directly, instead of using add_index after the create_table call:

create_table :products do |t|
  t.references :category, :index => true
end

will create an index identical to calling add_index :products, :category_id.

NOTE: The references helper does not actually create foreign key constraints for you. You will need to use execute or a plugin that adds foreign key support.

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

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 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).

Using the change Method

The change method removes the need to write both up and down methods in those cases that Rails knows how to revert the changes automatically. Currently, the change method supports only these migration definitions:

  • add_column
  • add_index
  • add_timestamps
  • create_table
  • remove_timestamps
  • rename_column
  • rename_index
  • rename_table

If you're going to need to use any other methods, you'll have to write the up and down methods instead of using the change method.

Using the up/down Methods

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. For example,

class ExampleMigration < ActiveRecord::Migration
  def up
    create_table :products do |t|
      t.references :category
    end
    #add a foreign key
    execute <<-SQL
      ALTER TABLE products
        ADD CONSTRAINT fk_products_categories
        FOREIGN KEY (category_id)
        REFERENCES categories(id)
    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 products
        DROP FOREIGN KEY fk_products_categories
    SQL
    drop_table :products
  end
end

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 from your down method. If someone tries to revert your migration, an error message will be displayed saying that it can't be done.

Running Migrations

Rails provides a set of rake tasks to work with migrations which boil down to running 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 up or change 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 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 (up, down or change) 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

$ rake db:migrate VERSION=20080906120000

If version 20080906120000 is greater than the current version (i.e., it is migrating upwards), this will run the 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.

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

$ rake db:rollback

This will run the down method from the latest migration. If you need to undo several migrations you can provide a STEP parameter:

$ rake db:rollback STEP=3

will run the down method from 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

$ 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.

Resetting the Database

The rake db:reset task will drop the database, recreate it and load the current schema into it.

NOTE: This is not the same as running all the migrations - see the section on schema.rb.

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 up or down method invoked, for example,

$ rake db:migrate:up VERSION=20080906120000

will run the up method from the 20080906120000 migration. 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.

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:

$ rake db:migrate RAILS_ENV=test

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:

Method Purpose
suppress_messages Takes a block as an argument and suppresses any output generated by the block.
say Takes a message argument and outputs it as is. A second boolean argument can be passed to specify whether to indent or not.
say_with_time 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.

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
      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.

Using Models in Your Migrations

When creating or updating data in a migration it is often tempting to use one of your models. After all, they exist to provide easy access to the underlying data. This can be done, but some caution should be observed.

For example, problems occur when the model uses database columns which are (1) not currently in the database and (2) will be created by this or a subsequent migration.

Consider this example, where Alice and Bob are working on the same code base which contains a Product model:

Bob goes on vacation.

Alice creates a migration for the products table which adds a new column and initializes it. She also adds a validation to the Product model for the new column.

# db/migrate/20100513121110_add_flag_to_product.rb

class AddFlagToProduct < ActiveRecord::Migration
  def change
    add_column :products, :flag, :boolean
    Product.update_all :flag => false
  end
end
# app/model/product.rb

class Product < ActiveRecord::Base
  validates :flag, :presence => true
end

Alice adds a second migration which adds and initializes another column to the products table and also adds a validation to the Product model for the new column.

# db/migrate/20100515121110_add_fuzz_to_product.rb

class AddFuzzToProduct < ActiveRecord::Migration
  def change
    add_column :products, :fuzz, :string
    Product.update_all :fuzz => 'fuzzy'
  end
end
# app/model/product.rb

class Product < ActiveRecord::Base
  validates :flag, :fuzz, :presence => true
end

Both migrations work for Alice.

Bob comes back from vacation and:

  • Updates the source - which contains both migrations and the latest version of the Product model.
  • Runs outstanding migrations with rake db:migrate, which includes the one that updates the Product model.

The migration crashes because when the model attempts to save, it tries to validate the second added column, which is not in the database when the first migration runs:

rake aborted!
An error has occurred, this and all later migrations canceled:

undefined method `fuzz' for #<Product:0x000001049b14a0>

A fix for this is to create a local model within the migration. This keeps Rails from running the validations, so that the migrations run to completion.

When using a faux model, it's a good idea to call Product.reset_column_information to refresh the ActiveRecord cache for the Product model prior to updating data in the database.

If Alice had done this instead, there would have been no problem:

# db/migrate/20100513121110_add_flag_to_product.rb

class AddFlagToProduct < ActiveRecord::Migration
  class Product < ActiveRecord::Base
  end

  def change
    add_column :products, :flag, :boolean
    Product.reset_column_information
    Product.update_all :flag => false
  end
end
# db/migrate/20100515121110_add_fuzz_to_product.rb

class AddFuzzToProduct < ActiveRecord::Migration
  class Product < ActiveRecord::Base
  end

  def change
    add_column :products, :fuzz, :string
    Product.reset_column_information
    Product.update_all :fuzz => 'fuzzy'
  end
end

Schema Dumping and You

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.

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 foreign key constraints, 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 the PostgreSQL RDBMS, 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.

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.

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 foreign key 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 any tools for working directly with such features, the execute method can be used to execute arbitrary SQL. You could also use some plugin like foreigner which add foreign key support to Active Record (including support for dumping foreign keys in db/schema.rb).

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