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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 a Ruby Domain Specific Language (DSL) to describe changes to your tables.

After reading this guide, you will know:

Migration Overview

Migrations are a convenient way to evolve your database schema over time in a reproducible 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. We recommend that you read the guides for Active Record Basics and the Active Record Associations to learn more about some of the concepts mentioned here.

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 indexes. 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. Read more about how Rails knows which migration in the timeline to run.

Active Record updates your db/schema.rb file to match the up-to-date structure of your database. Here's an example of a migration:

# db/migrate/20240502100843_create_products.rb
class CreateProducts < ActiveRecord::Migration[8.1]
  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.

# db/schema.rb
ActiveRecord::Schema[8.1].define(version: 2024_05_02_100843) do
  # These are extensions that must be enabled in order to support this database
  enable_extension "plpgsql"

  create_table "products", force: :cascade do |t|
    t.string "name"
    t.text "description"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end
end

We define the change that we want to happen moving forward in time. Before this migration is run, there will be no table. After it is run, 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. Read more about rolling back migrations in the Rolling Back section.

After defining the change that we want to occur moving forward in time, it's essential to consider the reversibility of the migration. While Active Record can manage the forward progression of the migration, ensuring the creation of the table, the concept of reversibility becomes crucial. With reversible migrations, not only does the migration create the table when applied, but it also enables smooth rollback functionality. In case of reverting the migration above, Active Record intelligently handles the removal of the table, maintaining database consistency throughout the process. See the Reversing Migrations section for more details.

Generating Migration Files

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, it contains 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, 20240502100843_create_products.rb should define class CreateProducts and 20240502101659_add_details_to_products.rb should define class 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 generating a file yourself, be aware of its position in the order. You can read more about how the timestamps are used in the Rails Migration Version Control section.

When generating a migration, Active Record automatically prepends the current timestamp to the file name of the migration. For example, running the command below will create an empty migration file whereby the filename is made up of a timestamp prepended to the underscored name of the migration.

$ bin/rails generate migration AddPartNumberToProducts

# db/migrate/20240502101659_add_part_number_to_products.rb
class AddPartNumberToProducts < ActiveRecord::Migration[8.1]
  def change
  end
end

The generator can do much more than prepend a timestamp to the file name. Based on naming conventions and additional (optional) arguments it can also start fleshing out the migration.

The following sections will cover the various ways you can create migrations based on conventions and additional arguments.

Creating a New Table

When you want to create a new table in your database, you can use a migration with the format "CreateXXX" followed by a list of column names and types. This will generate a migration file that sets up the table with the specified columns.

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

generates

class CreateProducts < ActiveRecord::Migration[8.1]
  def change
    create_table :products do |t|
      t.string :name
      t.string :part_number

      t.timestamps
    end
  end
end

The generated file with its contents is just a starting point, and you can add or remove from it as you see fit by editing the db/migrate/YYYYMMDDHHMMSS_create_products.rb file.

Adding Columns

When you want to add a new column to an existing table in your database, you can use a migration with the format "AddColumnToTable" followed by a list of column names and types. This will generate a migration file containing the appropriate add_column statements.

$ bin/rails generate migration AddPartNumberToProducts part_number:string

This will generate the following migration:

class AddPartNumberToProducts < ActiveRecord::Migration[8.1]
  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

This will generate the appropriate add_column and add_index statements:

class AddPartNumberToProducts < ActiveRecord::Migration[8.1]
  def change
    add_column :products, :part_number, :string
    add_index :products, :part_number
  end
end

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

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

This will generate a schema migration which adds two additional columns to the products table.

class AddDetailsToProducts < ActiveRecord::Migration[8.1]
  def change
    add_column :products, :part_number, :string
    add_column :products, :price, :decimal
  end
end

Removing Columns

Similarly, if the migration name is of the form "RemoveColumnFromTable" and is followed by a list of column names and types then a migration containing the appropriate remove_column statements will be created.

$ bin/rails generate migration RemovePartNumberFromProducts part_number:string

This will generate the appropriate remove_column statements:

class RemovePartNumberFromProducts < ActiveRecord::Migration[8.1]
  def change
    remove_column :products, :part_number, :string
  end
end

Creating Associations

Active Record associations are used to define relationships between different models in your application, allowing them to interact with each other through their relationships and making it easier to work with related data. To learn more about associations, you can refer to the Association Basics guide.

One common use case for associations is creating foreign key references between tables. The generator accepts column types such as references to facilitate this process. References are a shorthand for creating columns, indexes, foreign keys, or even polymorphic association columns.

For example,

$ bin/rails generate migration AddUserRefToProducts user:references

generates the following add_reference call:

class AddUserRefToProducts < ActiveRecord::Migration[8.1]
  def change
    add_reference :products, :user, null: false, foreign_key: true
  end
end

The above migration creates a foreign key called user_id in the products table, where user_id is a reference to the id column in the users table. It also creates an index for the user_id column. The schema looks as follows:

create_table "products", force: :cascade do |t|
  t.bigint "user_id", null: false
  t.index ["user_id"], name: "index_products_on_user_id"
end

belongs_to is an alias of references, so the above could be alternatively written as:

$ bin/rails generate migration AddUserRefToProducts user:belongs_to

generating a migration and schema that is the same as above.

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

$ bin/rails generate migration CreateJoinTableUserProduct user product

will produce the following migration:

class CreateJoinTableUserProduct < ActiveRecord::Migration[8.1]
  def change
    create_join_table :users, :products do |t|
      # t.index [:user_id, :product_id]
      # t.index [:product_id, :user_id]
    end
  end
end

Other Generators that Create Migrations

In addition to the migration generator, the model, resource, 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

This will create a migration that looks like this:

class CreateProducts < ActiveRecord::Migration[8.1]
  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.

Passing Modifiers

When generating migrations, you can pass commonly used type modifiers directly on the command line. These modifiers, enclosed by curly braces and following the field type, allow you to tailor the characteristics of your database columns without needing to manually edit the migration file afterward.

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[8.1]
  def change
    add_column :products, :price, :decimal, precision: 5, scale: 2
    add_reference :products, :supplier, polymorphic: true
  end
end

NOT NULL constraints can be imposed from the command line using the ! shortcut:

$ bin/rails generate migration AddEmailToUsers email:string!

will produce this migration

class AddEmailToUsers < ActiveRecord::Migration[8.1]
  def change
    add_column :users, :email, :string, null: false
  end
end

TIP: For further help with generators, run bin/rails generate --help. Alternatively, you can also run bin/rails generate model --help or bin/rails generate migration --help for help with specific generators.

Updating Migrations

Once you have created your migration file using one of the generators from the above section, you can update the generated migration file in the db/migrate folder to define further changes you want to make to your database schema.

Creating a Table

The create_table method is one of the most fundamental migration type, but most of the time, will be generated for you from using a model, resource, or scaffold generator. A typical use would be

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

This method creates a products table with a column called name.

Associations

If you're creating a table for a model that has an association, you can use the :references type to create the appropriate column type. For example:

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

This will create a category_id column. Alternatively, you can use belongs_to as an alias for references:

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

You can also specify the column type and index creation using the :polymorphic option:

create_table :taggings do |t|
  t.references :taggable, polymorphic: true
end

This will create taggable_id, taggable_type columns and the appropriate indexes.

Primary Keys

By default, create_table will implicitly create a primary key called id for you. You can change the name of the column with the :primary_key option, like below:

class CreateUsers < ActiveRecord::Migration[8.1]
  def change
    create_table :users, primary_key: "user_id" do |t|
      t.string :username
      t.string :email
      t.timestamps
    end
  end
end

This will yield the following schema:

create_table "users", primary_key: "user_id", force: :cascade do |t|
  t.string "username"
  t.string "email"
  t.datetime "created_at", precision: 6, null: false
  t.datetime "updated_at", precision: 6, null: false
end

You can also pass an array to :primary_key for a composite primary key. Read more about composite primary keys.

class CreateUsers < ActiveRecord::Migration[8.1]
  def change
    create_table :users, primary_key: [:id, :name] do |t|
      t.string :name
      t.string :email
      t.timestamps
    end
  end
end

If you don't want a primary key at all, you can pass the option id: false.

class CreateUsers < ActiveRecord::Migration[8.1]
  def change
    create_table :users, id: false do |t|
      t.string :username
      t.string :email
      t.timestamps
    end
  end
end

Database Options

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

This will append ENGINE=BLACKHOLE to the SQL statement used to create the table.

An index can be created on the columns created within the create_table block by passing index: true or an options hash to the :index option:

create_table :users do |t|
  t.string :name, index: true
  t.string :email, index: { unique: true, name: "unique_emails" }
end

Comments

You can pass the :comment option with any description for the table that will be stored in the database itself and can be viewed with database administration tools, such as MySQL Workbench or PgAdmin III. Comments can help team members to better understand the data model and to generate documentation in applications with large databases. Currently only the MySQL and PostgreSQL adapters support comments.

class AddDetailsToProducts < ActiveRecord::Migration[8.1]
  def change
    add_column :products, :price, :decimal, precision: 8, scale: 2, comment: "The price of the product in USD"
    add_column :products, :stock_quantity, :integer, comment: "The current stock quantity of the product"
  end
end

Creating a Join Table

The migration method create_join_table creates an HABTM (has and belongs to many) join table. A typical use would be:

create_join_table :products, :categories

This migration will create a categories_products table with two columns called category_id and product_id.

These columns have the option :null set to false by default, meaning that you must provide a value in order to save a record to this table. This can be overridden by specifying the :column_options option:

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

By default, the name of the join table comes from the union of the first two arguments provided to create_join_table, in lexical order. In this case, the table would be named categories_products.

WARNING: The precedence between model names is calculated using the <=> operator for String. This means that if the strings are of different lengths, and the strings are equal when compared up to the shortest length, then the longer string is considered of higher lexical precedence than the shorter one. For example, one would expect the tables "paper_boxes" and "papers" to generate a join table name of "papers_paper_boxes" because of the length of the name "paper_boxes", but it in fact generates a join table name of "paper_boxes_papers" (because the underscore '_' is lexicographically less than 's' in common encodings).

To customize the name of the table, provide a :table_name option:

create_join_table :products, :categories, table_name: :categorization

This creates a join table with the name categorization.

Also, create_join_table accepts a block, which you can use to add indices (which are not created by default) or any additional columns you so choose.

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

Changing Tables

If you want to change an existing table in place, there is change_table.

It is used in a similar fashion to create_table but the object yielded inside the block has access to a number of special functions, 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

This migration will remove the description and name columns, create a new string column called part_number and add an index on it. Finally, it renames the upccode column to upc_code.

Changing Columns

Similar to the remove_column and add_column methods we covered earlier, Rails also 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.

NOTE: The change_column command is irreversible. To ensure your migration can be safely reverted, you will need to provide your own reversible migration. See the Reversible Migrations section for more details.

Besides change_column, the change_column_null and change_column_default methods are used to change a null constraint and default values of a column.

change_column_default :products, :approved, from: true, to: false

This changes the default value of the :approved field from true to false. This change will only be applied to future records, any existing records do not change. Use change_column_null to change a null constraint.

change_column_null :products, :name, false

This sets :name field on products to a NOT NULL column. This change applies to existing records as well, so you need to make sure all existing records have a :name that is NOT NULL.

Setting the null constraint to true implies that column will accept a null value, otherwise the NOT NULL constraint is applied and a value must be passed in order to persist the record to the database.

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

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

NOTE: For add_column or change_column there is no option for adding indexes. They need to be added separately using add_index.

Some adapters may support additional options; see the adapter specific API docs for further information.

NOTE: default cannot be specified via command line when generating migrations.

References

The add_reference method allows the creation of an appropriately named column acting as the connection between one or more associations.

add_reference :users, :role

This migration will create a foreign key column called role_id in the users table. role_id is a reference to the id column in the roles table. In addition, it creates an index for the role_id column, unless it is explicitly told not to do so with the index: false option.

INFO: See also the Active Record Associations guide to learn more.

The method add_belongs_to is an alias of add_reference.

add_belongs_to :taggings, :taggable, polymorphic: true

The polymorphic option will create two columns on the taggings table which can be used for polymorphic associations: taggable_type and taggable_id.

INFO: See this guide to learn more about polymorphic associations.

A foreign key can be created with the foreign_key option.

add_reference :users, :role, foreign_key: true

For more add_reference options, visit the API documentation.

References can also be removed:

remove_reference :products, :user, foreign_key: true, index: false

Foreign Keys

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

add_foreign_key :articles, :authors

The add_foreign_key call adds a new constraint to the articles table. The constraint guarantees that a row in the authors table exists where the id column matches the articles.author_id to ensure all reviewers listed in the articles table are valid authors listed in the authors table.

NOTE: When using references in a migration, you are creating a new column in the table and you'll have the option to add a foreign key using foreign_key: true to that column. However, if you want to add a foreign key to an existing column, you can use add_foreign_key.

If the column name of the table to which we're adding the foreign key cannot be derived from the table with the referenced primary key then you can use the :column option to specify the column name. Additionally, you can use the :primary_key option if the referenced primary key is not :id.

For example, to add a foreign key on articles.reviewer referencing authors.email:

add_foreign_key :articles, :authors, column: :reviewer, primary_key: :email

This will add a constraint to the articles table that guarantees a row in the authors table exists where the email column matches the articles.reviewer field.

Several other options such as name, on_delete, if_not_exists, validate, and deferrable are supported by add_foreign_key.

Foreign keys can also be removed using remove_foreign_key:

# 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

NOTE: Active Record only supports single column foreign keys. execute and structure.sql are required to use composite foreign keys. See Schema Dumping and You.

Composite Primary Keys

Sometimes a single column's value isn't enough to uniquely identify every row of a table, but a combination of two or more columns does uniquely identify it. This can be the case when using a legacy database schema without a single id column as a primary key, or when altering schemas for sharding or multitenancy.

You can create a table with a composite primary key by passing the :primary_key option to create_table with an array value:

class CreateProducts < ActiveRecord::Migration[8.1]
  def change
    create_table :products, primary_key: [:customer_id, :product_sku] do |t|
      t.integer :customer_id
      t.string :product_sku
      t.text :description
    end
  end
end

INFO: Tables with composite primary keys require passing array values rather than integer IDs to many methods. See also the Active Record Composite Primary Keys guide to learn more.

Execute SQL

If the helpers provided by Active Record aren't enough, you can use the execute method to execute SQL commands. For example,

class UpdateProductPrices < ActiveRecord::Migration[8.1]
  def up
    execute "UPDATE products SET price = 'free'"
  end

  def down
    execute "UPDATE products SET price = 'original_price' WHERE price = 'free';"
  end
end

In this example, we're updating the price column of the products table to 'free' for all records.

WARNING: Modifying data directly in migrations should be approached with caution. Consider if this is the best approach for your use case, and be aware of potential drawbacks such as increased complexity and maintenance overhead, risks to data integrity and database portability. See the Data Migrations documentation for more details.

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.

For methods available regarding the object yielded by create_table, see ::ActiveRecord::ConnectionAdapters::TableDefinition.

And for the object yielded by change_table, see ::ActiveRecord::ConnectionAdapters::Table.

Using the change Method

The change method is the primary way of writing migrations. It works for the majority of cases in which Active Record knows how to reverse a migration's actions automatically. Below are some of the actions that change supports:

change_table is also reversible, as long as the block only calls reversible operations like the ones listed above.

If you need to use any other methods, you should use reversible or write the up and down methods instead of using the change method.

Using reversible

If you'd like for a migration to do something that Active Record doesn't know how to reverse, then you can use reversible to specify what to do when running a migration and what else to do when reverting it.

class ChangeProductsPrice < ActiveRecord::Migration[8.1]
  def change
    reversible do |direction|
      change_table :products do |t|
        direction.up   { t.change :price, :string }
        direction.down { t.change :price, :integer }
      end
    end
  end
end

This migration will change the type of the price column to a string, or back to an integer when the migration is reverted. Notice the block being passed to direction.up and direction.down respectively.

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

class ChangeProductsPrice < ActiveRecord::Migration[8.1]
  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

Additionally, reversible is useful when executing raw SQL queries or performing database operations that do not have a direct equivalent in ActiveRecord methods. You can use reversible to specify what to do when running a migration and what else to do when reverting it. For example:

class ExampleMigration < ActiveRecord::Migration[8.1]
  def change
    create_table :distributors do |t|
      t.string :zipcode
    end

    reversible do |direction|
      direction.up do
        # create a distributors view
        execute <<-SQL
          CREATE VIEW distributors_view AS
          SELECT id, zipcode
          FROM distributors;
        SQL
      end
      direction.down do
        execute <<-SQL
          DROP VIEW distributors_view;
        SQL
      end
    end

    add_column :users, :address, :string
  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 users.address column is removed and before the distributors table is dropped.

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 perform 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[8.1]
  def up
    create_table :distributors do |t|
      t.string :zipcode
    end

    # create a distributors view
    execute <<-SQL
      CREATE VIEW distributors_view AS
      SELECT id, zipcode
      FROM distributors;
    SQL

    add_column :users, :address, :string
  end

  def down
    remove_column :users, :address

    execute <<-SQL
      DROP VIEW distributors_view;
    SQL

    drop_table :distributors
  end
end

Throwing an error to prevent reverts

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.

class IrreversibleMigrationExample < ActiveRecord::Migration[8.1]
  def up
    drop_table :example_table
  end

  def down
    raise ActiveRecord::IrreversibleMigration, "This migration cannot be reverted because it destroys data."
  end
end

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

Reverting Previous Migrations

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

require_relative "20121212123456_example_migration"

class FixupExampleMigration < ActiveRecord::Migration[8.1]
  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 that a Distributors view is no longer needed.

class DontUseDistributorsViewMigration < ActiveRecord::Migration[8.1]
  def change
    revert do
      # copy-pasted code from ExampleMigration
      create_table :distributors do |t|
        t.string :zipcode
      end

      reversible do |direction|
        direction.up do
          # create a distributors view
          execute <<-SQL
            CREATE VIEW distributors_view AS
            SELECT id, zipcode
            FROM distributors;
          SQL
        end
        direction.down do
          execute <<-SQL
            DROP VIEW distributors_view;
          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:

  1. Reverse the order of create_table and reversible.
  2. Replace create_table with drop_table.
  3. Finally, replace up with down and vice-versa.

This is all taken care of by revert.

Running Migrations

Rails provides a set of commands to run certain sets of migrations.

The very first migration related rails command you will use will probably be bin/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.

Note that running the db:migrate command also invokes the db:schema:dump command, 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 20240428000000 run:

$ bin/rails db:migrate VERSION=20240428000000

If version 20240428000000 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 20240428000000, 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, 20240428000000.

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/rails 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/rails db:rollback STEP=3

The last 3 migrations will be reverted.

In some cases where you modify a local migration and would like to rollback that specific migration before migrating back up again, you can use the db:migrate:redo command. As with the db:rollback command, you can use the STEP parameter if you need to go more than one version back, for example:

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

NOTE: You could get the same result using db:migrate. However, these are there for convenience so that you do not need to explicitly specify the version to migrate to.

Transactions

In databases that support DDL transactions, changing the schema in a single transaction, each migration is wrapped in a transaction.

INFO: A transaction ensures that if a migration fails partway through, any changes that were successfully applied are rolled back, maintaining database consistency. This means that either all operations within the transaction are executed successfully, or none of them are, preventing the database from being left in an inconsistent state if an error occurs during the transaction.

If the database does not support DDL transactions with statements that change the schema, then when a migration fails, the parts of it that have succeeded will not be rolled back. You will have to rollback the changes manually.

There are queries that you can’t execute inside a transaction though, and for these situations you can turn the automatic transactions off with disable_ddl_transaction!:

class ChangeEnum < ActiveRecord::Migration[8.1]
  disable_ddl_transaction!

  def up
    execute "ALTER TYPE model_size ADD VALUE 'new_value'"
  end
end

NOTE: Remember that you can still open your own transactions, even if you are in a Migration with self.disable_ddl_transaction!.

Setting Up the Database

The bin/rails db:setup command will create the database, load the schema, and initialize it with the seed data.

Preparing the Database

The bin/rails db:prepare command is similar to bin/rails db:setup, but it operates idempotently, so it can safely be called several times, but it will only perform the necessary tasks once.

Once the database and tables exist, the db:prepare task will not try to reload the seed data, even if the previously loaded seed data or the existing seed file have been altered or deleted. To reload the seed data, you can manually run bin/rails db:seed.

NOTE: This task will only load seeds if one of the databases or tables created is a primary database for the environment or is configured with seeds: true.

Resetting the Database

The bin/rails db:reset command will drop the database and set it up again. This is functionally equivalent to bin/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 db/schema.rb or db/structure.sql file. If a migration can't be rolled back, bin/rails db:reset may not help you. To find out more about dumping the schema see Schema Dumping and You section.

Running Specific Migrations

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

$ bin/rails db:migrate:up VERSION=20240428000000

By running this command the change method (or the up method) will be executed for the migration with the version "20240428000000".

First, this command will check whether the migration exists and if it has already been performed and if so, it will do nothing.

If the version specified does not exist, Rails will throw an exception.

$ bin/rails db:migrate VERSION=00000000000000
rails aborted!
ActiveRecord::UnknownMigrationVersionError:

No migration with version number 00000000000000.

Running Migrations in Different Environments

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:

$ bin/rails 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, take the following migration:

class CreateProducts < ActiveRecord::Migration[8.1]
  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

This will generate 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 bin/rails db:migrate VERBOSE=false will suppress all output.

Rails Migration Version Control

Rails keeps track of which migrations have been run through the schema_migrations table in the database. When you run a migration, Rails inserts a row into the schema_migrations table with the version number of the migration, stored in the version column. This allows Rails to determine which migrations have already been applied to the database.

For example, if you have a migration file named 20240428000000_create_users.rb, Rails will extract the version number (20240428000000) from the filename and insert it into the schema_migrations table after the migration has been successfully executed.

You can view the contents of the schema_migrations table directly in your database management tool or by using Rails console:

rails dbconsole

Then, within the database console, you can query the schema_migrations table:

SELECT * FROM schema_migrations;

This will show you a list of all migration version numbers that have been applied to the database. Rails uses this information to determine which migrations need to be run when you run rails db:migrate or rails db:migrate:up commands.

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 bin/rails db:migrate. You must rollback the migration (for example with bin/rails db:rollback), edit your migration, and then run bin/rails db:migrate to run the corrected version.

In general, editing existing migrations that have been already committed to source control 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.

However, editing a freshly generated migration that has not yet been committed to source control (or, more generally, has not been propagated beyond your development machine) is common.

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).

Schema Dumping and You

What are Schema Files for?

Migrations, mighty as they may be, are not the authoritative source for your database schema. Your database remains the source of truth.

By default, Rails generates db/schema.rb which attempts to capture the current state of your database schema.

It tends to be faster and less error prone to create a new instance of your application's database by loading the schema file via bin/rails db:schema:load than it is to replay the entire migration history. Old migrations may fail to apply correctly if those migrations use changing external dependencies or rely on application code which evolves separately from your migrations.

TIP: 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.

Types of Schema Dumps

The format of the schema dump generated by Rails is controlled by the config.active_record.schema_format setting defined in config/application.rb. By default, the format is :ruby, or alternatively can be set to :sql.

Using the default :ruby schema

When :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[8.1].define(version: 2008_09_06_171750) 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.

Using the :sql schema dumper

However, db/schema.rb cannot express everything your database may support such as triggers, sequences, stored procedures, etc.

While migrations may use execute to create database constructs that are not supported by the Ruby migration DSL, these constructs may not be able to be reconstituted by the schema dumper.

If you are using features like these, you should set the schema format to :sql in order to get an accurate schema file that is useful to create new database instances.

When the schema format is set to :sql, the database structure will be dumped using a tool specific to the database 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.

To load the schema from db/structure.sql, run bin/rails db:schema:load. Loading this file is done by executing the SQL statements it contains. By definition, this will create a perfect copy of the database's structure.

Schema Dumps and Source Control

Because schema files are commonly used to create new databases, it is strongly recommended that you check your schema file into source control.

Merge conflicts can occur in your schema file when two branches modify schema. To resolve these conflicts run bin/rails db:migrate to regenerate the schema file.

INFO: Newly generated Rails apps will already have the migrations folder included in the git tree, so all you have to do is be sure to add any new migrations you add and commit them.

Active Record and Referential Integrity

The Active Record pattern suggests that intelligence should primarily reside in your models rather than in the database. Consequently, features like triggers or constraints, which delegate some of that intelligence back into the database, are not always favored.

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.

In practice, foreign key constraints and unique indexes are generally considered safer when enforced at the database level. Although Active Record does not provide direct support for working with these database-level features, you can still use the execute method to run arbitrary SQL commands.

It's worth emphasizing that while the Active Record pattern emphasizes keeping intelligence within models, neglecting to implement foreign keys and unique constraints at the database level can potentially lead to integrity issues. Therefore, it's advisable to complement the AR pattern with database-level constraints where appropriate. These constraints should have their counterparts explicitly defined in your code using associations and validations to ensure data integrity across both application and database layers.

Migrations and Seed Data

The main purpose of the 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.

class AddInitialProducts < ActiveRecord::Migration[8.1]
  def up
    5.times do |i|
      Product.create(name: "Product ##{i}", description: "A product.")
    end
  end

  def down
    Product.delete_all
  end
end

To add initial data after a database is created, Rails has a built-in 'seeds' feature that speeds up the process. This is especially useful when reloading the database frequently in development and test environments, or when setting up initial data for production.

To get started with this feature, open up db/seeds.rb and add some Ruby code, then run bin/rails db:seed.

NOTE: The code here should be idempotent so that it can be executed at any point in every environment.

["Action", "Comedy", "Drama", "Horror"].each do |genre_name|
  MovieGenre.find_or_create_by!(name: genre_name)
end

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

Old Migrations

The db/schema.rb or db/structure.sql is a snapshot of the current state of your database and is the authoritative source for rebuilding that database. This makes it possible to delete or prune old migration files.

When you delete migration files in the db/migrate/ directory, any environment where bin/rails db:migrate was run when those files still existed will hold a reference to the migration timestamp specific to them inside an internal Rails database table named schema_migrations. You can read more about this in the Rails Migration Version Control section.

If you run the bin/rails db:migrate:status command, which displays the status (up or down) of each migration, you should see ********** NO FILE ********** displayed next to any deleted migration file which was once executed on a specific environment but can no longer be found in the db/migrate/ directory.

Migrations from Engines

When dealing with migrations from Engines, there's a caveat to consider. Rake tasks to install migrations from engines are idempotent, meaning they will have the same result no matter how many times they are called. Migrations present in the parent application due to a previous installation are skipped, and missing ones are copied with a new leading timestamp. If you deleted old engine migrations and ran the install task again, you'd get new files with new timestamps, and db:migrate would attempt to run them again.

Thus, you generally want to preserve migrations coming from engines. They have a special comment like this:

# This migration comes from blorgh (originally 20210621082949)

Miscellaneous

Using UUIDs instead of IDs for Primary Keys

By default, Rails uses auto-incrementing integers as primary keys for database records. However, there are scenarios where using Universally Unique Identifiers (UUIDs) as primary keys can be advantageous, especially in distributed systems or when integration with external services is necessary. UUIDs provide a globally unique identifier without relying on a centralized authority for generating IDs.

Enabling UUIDs in Rails

Before using UUIDs in your Rails application, you'll need to ensure that your database supports storing them. Additionally, you may need to configure your database adapter to work with UUIDs.

NOTE: If you are using a version of PostgreSQL prior to 13, you may still need to enable the pgcrypto extension to access the gen_random_uuid() function.

  1. Rails Configuration

    In your Rails application configuration file (config/application.rb), add the following line to configure Rails to generate UUIDs as primary keys by default:

    config.generators do |g|
  g.orm :active_record, primary_key_type: :uuid
end

    This setting instructs Rails to use UUIDs as the default primary key type for ActiveRecord models.

  2. Adding References with UUIDs:

    When creating associations between models using references, ensure that you specify the data type as :uuid to maintain consistency with the primary key type. For example:

    create_table :posts, id: :uuid do |t|
  t.references :author, type: :uuid, foreign_key: true
  # Other columns...
  t.timestamps
end

    In this example, the author_id column in the posts table references the id column of the authors table. By explicitly setting the type to :uuid, you ensure that the foreign key column matches the data type of the primary key it references. Adjust the syntax accordingly for other associations and databases.

  3. Migration Changes

    When generating migrations for your models, you'll notice that it specifies the id to be of type uuid:

      $ bin/rails g migration CreateAuthors

    class CreateAuthors < ActiveRecord::Migration[8.1]
  def change
    create_table :authors, id: :uuid do |t|
      t.timestamps
    end
  end
end

    which results in the following schema:

    create_table "authors", id: :uuid, default: -> { "gen_random_uuid()" }, force: :cascade do |t|
  t.datetime "created_at", precision: 6, null: false
  t.datetime "updated_at", precision: 6, null: false
end

    In this migration, the id column is defined as a UUID primary key with a default value generated by the gen_random_uuid() function.

UUIDs are guaranteed to be globally unique across different systems, making them suitable for distributed architectures. They also simplify integration with external systems or APIs by providing a unique identifier that doesn't rely on centralized ID generation, and unlike auto-incrementing integers, UUIDs don't expose information about the total number of records in a table, which can be beneficial for security purposes.

However, UUIDs can also impact performance due to their size and are harder to index. UUIDs will have worse performance for writes and reads compared with integer primary keys and foreign keys.

NOTE: Therefore, it's essential to evaluate the trade-offs and consider the specific requirements of your application before deciding to use UUIDs as primary keys.

Data Migrations

Data migrations involve transforming or moving data within your database. In Rails, it is generally not advised to perform data migrations using migration files. Here’s why:

Instead, consider using the maintenance_tasks gem. This gem provides a framework for creating and managing data migrations and other maintenance tasks in a way that is safe and easy to manage without interfering with schema migrations.