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Active Record Migrations

Migrations are a feature of Active Record that allows you to evolve your database schema over time. Rather than write schema modifications in pure SQL, migrations allow you to use an easy Ruby DSL to describe changes to your tables.

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.
  • How migrations relate to schema.rb.

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
    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 up and down instead of change:

class ChangeProductsPrice < ActiveRecord::Migration
  def up
    change_table :products do |t|
      t.string :price, null: false
    end
  end

  def down
    change_table :products do |t|
      t.integer :price, null: false
    end
  end
end

Creating a Migration

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.

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

$ 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.

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

$ 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
    end
  end
end

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

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

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).

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.

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:

Products.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 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 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_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 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. 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 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 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. 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 'schema dumping and you.'

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.

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.

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 local 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

There are other ways in which the above example could have gone badly.

For example, imagine that Alice creates a migration that selectively updates the description field on certain products. She runs the migration, commits the code, and then begins working on the next feature, which is to add a new column fuzz to the products table.

She creates two migrations for this new feature, one which adds the new column, and a second which selectively updates the fuzz column based on other product attributes.

These migrations run just fine, but when Bob comes back from his vacation and calls rake db:migrate to run all the outstanding migrations, he gets a subtle bug: The descriptions have defaults, and the fuzz column is present, but fuzz is nil on all products.

The solution is again to use Product.reset_column_information before referencing the Product model in a migration, ensuring the Active Record's knowledge of the table structure is current before manipulating data in those records.

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 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.

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).

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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