rails/guides/source/active_record_migrations.md

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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 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 rails commands 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 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[7.2]
  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, each migration is 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.

NOTE: There are certain queries that can't run inside a transaction. If your adapter supports DDL transactions you can use disable_ddl_transaction! to disable them for a single migration.

Making the Irreversible Possible

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

class ChangeProductsPrice < ActiveRecord::Migration[7.2]
  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[7.2]
  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

INFO: More on reversible later.

Generating Migrations

Creating a Standalone Migration

Migrations are stored as files in the db/migrate directory, one for each migration class. The name of the file is of the form YYYYMMDDHHMMSS_create_products.rb, that is to say a UTC timestamp identifying the migration followed by an underscore followed by the name of the migration. The name of the migration class (CamelCased version) should match the latter part of the file name. For example 20080906120000_create_products.rb should define class CreateProducts and 20080906120001_add_details_to_products.rb should define AddDetailsToProducts. Rails uses this timestamp to determine which migration should be run and in what order, so if you're copying a migration from another application or generate a file yourself, be aware of its position in the order.

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

$ bin/rails generate migration AddPartNumberToProducts

This will create an appropriately named empty migration:

class AddPartNumberToProducts < ActiveRecord::Migration[7.2]
  def change
  end
end

This 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.

Adding New Columns

If the migration name is of the form "AddColumnToTable" or "RemoveColumnFromTable" and is followed by a list of column names and types then a migration containing the appropriate add_column and remove_column statements will be created.

$ bin/rails generate migration AddPartNumberToProducts part_number:string

This will generate the following migration:

class AddPartNumberToProducts < ActiveRecord::Migration[7.2]
  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[7.2]
  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

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

class AddDetailsToProducts < ActiveRecord::Migration[7.2]
  def change
    add_column :products, :part_number, :string
    add_column :products, :price, :decimal
  end
end

Removing Columns

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

$ bin/rails generate migration RemovePartNumberFromProducts part_number:string

This generates the appropriate remove_column statements:

class RemovePartNumberFromProducts < ActiveRecord::Migration[7.2]
  def change
    remove_column :products, :part_number, :string
  end
end

Creating New Tables

If the migration name is of the form "CreateXXX" and is followed by a list of column names and types then a migration creating the table XXX with the columns listed will be generated. For example:

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

generates

class CreateProducts < ActiveRecord::Migration[7.2]
  def change
    create_table :products do |t|
      t.string :name
      t.string :part_number

      t.timestamps
    end
  end
end

As always, what has been generated for you is just a starting point. You can add or remove from it as you see fit by editing the db/migrate/YYYYMMDDHHMMSS_create_products.rb file.

Creating associations using references

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

$ bin/rails generate migration AddUserRefToProducts user:references

generates the following add_reference call:

class AddUserRefToProducts < ActiveRecord::Migration[7.2]
  def change
    add_reference :products, :user, foreign_key: true
  end
end

This migration will create a user_id column. References are a shorthand for creating columns, indexes, foreign keys, or even polymorphic association columns.

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

$ bin/rails generate migration CreateJoinTableCustomerProduct customer product

will produce the following migration:

class CreateJoinTableCustomerProduct < ActiveRecord::Migration[7.2]
  def change
    create_join_table :customers, :products do |t|
      # t.index [:customer_id, :product_id]
      # t.index [:product_id, :customer_id]
    end
  end
end

Model Generators

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[7.2]
  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

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

For instance, running:

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

will produce a migration that looks like this

class AddDetailsToProducts < ActiveRecord::Migration[7.2]
  def change
    add_column :products, :price, :decimal, precision: 5, scale: 2
    add_reference :products, :supplier, polymorphic: true
  end
end

TIP: Have a look at the generators help output (bin/rails generate --help) for further details.

Writing Migrations

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, 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.

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, or pass an array to :primary_key for a composite primary key. 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

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

Also, 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. It's highly recommended to specify comments in migrations for applications with large databases as it helps people to understand the data model and generate documentation. Currently only the MySQL and PostgreSQL adapters support comments.

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 alphabetical order.

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

create_join_table :products, :categories, table_name: :categorization

This ensure the name of the join table is categorization as requested.

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 adds 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. You should provide your own reversible migration, like we discussed before.

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

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

This sets :name field on products to a NOT NULL column and the default value of the :approved field from true to false. Both of these changes will only be applied to future transactions, any existing records do not apply.

When setting the null constraint to true, this means 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:

• comment Adds a comment for the column.
• collation Specifies the collation for a string or text column.
• default Allows to set a default value on the column. Note that if you are using a dynamic value (such as a date), the default will only be calculated the first time (i.e. on the date the migration is applied). Use nil for NULL.
• limit Sets the maximum number of characters for a string column and the maximum number of bytes for text/binary/integer columns.
• null Allows or disallows NULL values in the column.
• precision Specifies the precision for decimal/numeric/datetime/time columns.
• scale Specifies the scale for the decimal and numeric columns, representing the number of digits after the decimal point.

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: null and 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 role_id column in the users table. It creates an index for this column as well, unless explicitly told not to 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

This 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.

If the from_table column name cannot be derived from the to_table name, you can use the :column option. 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[7.2]
  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 Querying guide to learn more.

When Helpers aren't Enough

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

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

For more details and examples of individual methods, check the API documentation.

In particular the documentation for ActiveRecord::ConnectionAdapters::SchemaStatements, which provides the methods available in the change, up and down methods.

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:

• add_check_constraint
• add_column
• add_foreign_key
• add_index
• add_reference
• add_timestamps
• change_column_comment (must supply :from and :to options)
• change_column_default (must supply :from and :to options)
• change_column_null
• change_table_comment (must supply :from and :to options)
• create_join_table
• create_table
• disable_extension
• drop_join_table
• drop_table (must supply table creation options and block)
• enable_extension
• remove_check_constraint (must supply original constraint expression)
• remove_column (must supply original type and column options)
• remove_columns (must supply original type and column options)
• remove_foreign_key (must supply other table and original options)
• remove_index (must supply columns and original options)
• remove_reference (must supply original options)
• remove_timestamps (must supply original options)
• rename_column
• rename_index
• rename_table

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

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

Using reversible

Complex migrations may require processing that Active Record doesn't know how to reverse. You can use reversible to specify what to do when running a migration and what else to do when reverting it. For example:

class ExampleMigration < ActiveRecord::Migration[7.2]
  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[7.2]
  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.

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[7.2]
  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[7.2]
  def change
    revert do
      # copy-pasted code from ExampleMigration
      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 20080906120000 run:

$ bin/rails db:migrate VERSION=20080906120000

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

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.

The db:migrate:redo command is a shortcut for doing a rollback and then migrating back up again. 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

Neither of these rails commands do anything you could not do with db:migrate. They are there for convenience, since you do not need to explicitly specify the version to migrate to.

Setup 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.

• If the database has not been created yet, the command will run as the bin/rails db:setup does.
• If the database exists but the tables have not been created, the command will load the schema, run any pending migrations, dump the updated schema, and finally load the seed data.
• If both the database and tables exist but the seed data has not been loaded, the command will only load the seed data.
• If the database, tables, and seed data are all in place, the command will do nothing.

NOTE: Once the database, tables, and seed data are all established, the command 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.

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=20080906120000

By running this command the change method (or the up method) will be executed for the migration with the version "20080906120000".

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

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

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

No migration with version number zomg.

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[7.2]
  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.

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 is not a good idea. You will be creating extra work for yourself and your co-workers and cause major headaches if the existing version of the migration has already been run on production machines.

Instead, you should write a new migration that performs the changes you require. Editing a freshly generated migration that has not yet been committed to source control (or, more generally, which has not been propagated beyond your development machine) is relatively harmless.

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

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.

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[7.2].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 way claims that intelligence belongs in your models, not in the database. As such, features such as triggers or constraints, which push some of that intelligence back into the database, are not recommended.

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

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

Migrations and Seed Data

The main purpose of Rails' migration feature is to issue commands that modify the schema using a consistent process. Migrations can also be used to add or modify data. This is useful in an existing database that can't be destroyed and recreated, such as a production database.

class AddInitialProducts < ActiveRecord::Migration[7.2]
  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. This table is used to keep track of whether migrations have been executed in a specific environment.

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

There's a caveat, though with Engines. 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)