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SchemaAutoForeignKeys is part of the SchemaPlus family of Ruby on Rails ActiveRecord extension gems.


Many of us think that it should goes without saying that if you define a foreign key relation in your database you should also define a foreign key constraint.

Similarly, it should go without saying that if you have a foreign key constraint on a column, you should also have an index on that column.

And if you include the schema_auto_foreign_keys gem, these will also go without typing! schema_auto_foreign_keys simply turns on some default behavior in your migrations:

t.integer :user_id  # any column named xxxx_id defaults to...
t.integer :user_id, foreign_key: true, index: true

t.references :user # references defaults to...
t.references :user, foreign_key: true, index: true

t.belongs_to :user # belongs_to default to...
t.belongs_to :user, foreign_key: true, index: true

Note that schema_auto_foreign_keys depends on the schema_plus_foreign_keys and schema_plus_indexes gems, and so makes available their migration shortcuts.

There is actually one difference between an auto-created index and specifying index: true: if you don't specify anything, schema_auto_foreign_keys will maintain "ownership" of the auto-created index: It will remove the index if the foreign key gets removed; and it will rename the index if the table gets renamed.


If you need specific paramaters other than the default, you can of course specify them:

t.integer :user_id, index: :unique  # "has one" relationship between users and this 
t.integer :user_id, on_delete: :cascade

If you don't want a foreign key constraint (e.g. because "product_id" is a domain-level string rather than a foreign key), or an index just specify falsey:

t.integer :product_id, foreign_key: false # also implies index: false
t.integer :product_id, references: nil
t.integer :user_id, index: false


SchemaAutoForeignKeys adds two new entries to SchemaPlus::ForeignKeys' config:

SchemaPlus::ForeignKeys.setup do |config|
   config.auto_create = true # default for schema_auto_foreign_keys
   config.auto_index = true # default for schema_auto_foreign_keys

You can also configure the behavior per-table in a migration:

create_table :posts, foreign_keys: { auto_create: true, auto_index: true } do |t|
   t.integer :author_id


As usual:

gem "schema_auto_foreign_keys"                # in a Gemfile
gem.add_dependency "schema_auto_foreign_keys" # in a .gemspec


SchemaAutoForeignKeys is tested on:

  • ruby 2.3.1 with activerecord 4.2, using mysql2, sqlite3 or postgresql
  • ruby 2.3.1 with activerecord 5.0, using mysql2, sqlite3 or postgresql

Platform-specific Notes:

MySQL automatically creates indexes for foreign key constraints, so when used with MySQL, schema_auto_foreign_keys doesn't include the auto-index capability.

SQlite3 doesn't support renaming the auto-index whtn the table name changes.


  • 0.1.3 - AR5 (Rails 5) Support
  • 0.1.2 - Missing require
  • 0.1.1 - Explicit gem dependencies
  • 0.1.0 - Initial release, extracted from schema_plus 2.0.0.pre*

Development & Testing

Are you interested in contributing to SchemaAutoForeignKeys? Thanks! Please follow the standard protocol: fork, feature branch, develop, push, and issue pull request.

Some things to know about to help you develop and test:

  • schema_dev: SchemaAutoForeignKeys uses schema_dev to facilitate running rspec tests on the matrix of ruby, activerecord, and database versions that the gem supports, both locally and on travis-ci

    To to run rspec locally on the full matrix, do:

      $ schema_dev bundle install
      $ schema_dev rspec

    You can also run on just one configuration at a time; For info, see schema_dev --help or the schema_dev README.

    The matrix of configurations is specified in schema_dev.yml in the project root.

  • schema_monkey: SchemaAutoForeignKeys is implemented as a schema_monkey client, using schema_monkey's convention-based protocols for extending ActiveRecord and using middleware stacks.


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