gitlab-org--gitlab-foss/doc/development/foreign_keys.md

Foreign Keys & Associations

When adding an association to a model you must also add a foreign key. For example, say you have the following model:

class User < ActiveRecord::Base
  has_many :posts
end

Here you will need to add a foreign key on column posts.user_id. This ensures that data consistency is enforced on database level. Foreign keys also mean that the database can very quickly remove associated data (e.g. when removing a user), instead of Rails having to do this.

Adding Foreign Keys In Migrations

Foreign keys can be added concurrently using add_concurrent_foreign_key as defined in Gitlab::Database::MigrationHelpers. See the Migration Style Guide for more information.

Keep in mind that you can only safely add foreign keys to existing tables after you have removed any orphaned rows. The method add_concurrent_foreign_key does not take care of this so you'll need to do so manually.

Cascading Deletes

Every foreign key must define an ON DELETE clause, and in 99% of the cases this should be set to CASCADE.

Indexes

When adding a foreign key in PostgreSQL the column is not indexed automatically, thus you must also add a concurrent index. Not doing so will result in cascading deletes being very slow.

Dependent Removals

Don't define options such as dependent: :destroy or dependent: :delete when defining an association. Defining these options means Rails will handle the removal of data, instead of letting the database handle this in the most efficient way possible.

In other words, this is bad and should be avoided at all costs:

class User < ActiveRecord::Base
  has_many :posts, dependent: :destroy
end

Should you truly have a need for this it should be approved by a database specialist first.

You should also not define any before_destroy or after_destroy callbacks on your models unless absolutely required and only when approved by database specialists. For example, if each row in a table has a corresponding file on a file system it may be tempting to add a after_destroy hook. This however introduces non database logic to a model, and means we can no longer rely on foreign keys to remove the data as this would result in the filesystem data being left behind. In such a case you should use a service class instead that takes care of removing non database data.