Add a foreign key constraint to an existing column

Foreign keys ensure consistency between related database tables. The current database review process always encourages you to add foreign keys when creating tables that reference records from other tables.

Starting with Rails version 4, Rails includes migration helpers to add foreign key constraints to database tables. Before Rails 4, the only way for ensuring some level of consistency was the dependent option in the association definition. Ensuring data consistency on the application level could fail in some unfortunate cases, so we might end up with inconsistent data in the table. This mostly affects older tables, where we didn't have the framework support to ensure consistency on the database level. These data inconsistencies can cause unexpected application behavior or bugs.

Adding a foreign key to an existing database column requires database structure changes and potential data changes. In case the table is in use, we should always assume that there is inconsistent data.

To add a foreign key constraint to an existing column:

1. GitLab version N.M: Add a NOT VALID foreign key constraint to the column to ensure GitLab doesn't create inconsistent records.
2. GitLab version N.M: Add a data migration, to fix or clean up existing records.
3. GitLab version N.M+1: Validate the whole table by making the foreign key VALID.

Example

Consider the following table structures:

users table:

• id (integer, primary key)
• name (string)

emails table:

• id (integer, primary key)
• user_id (integer)
• email (string)

Express the relationship in ActiveRecord:

class User < ActiveRecord::Base
  has_many :emails
end

class Email < ActiveRecord::Base
  belongs_to :user
end

Problem: when the user is removed, the email records related to the removed user stays in the emails table:

user = User.find(1)
user.destroy

emails = Email.where(user_id: 1) # returns emails for the deleted user

Prevent invalid records

Add a NOT VALID foreign key constraint to the table, which enforces consistency on the record changes.

In the example above, you'd be still able to update records in the emails table. However, when you'd try to update the user_id with non-existent value, the constraint causes a database error.

Migration file for adding NOT VALID foreign key:

class AddNotValidForeignKeyToEmailsUser < Gitlab::Database::Migration[2.1]
  def up
    add_concurrent_foreign_key :emails, :users, column: :user_id, on_delete: :cascade, validate: false
  end

  def down
    remove_foreign_key_if_exists :emails, column: :user_id
  end
end

Adding a foreign key without validating it is a fast operation. It only requires a short lock on the table before being able to enforce the constraint on new data. We do still want to enable lock retries for high traffic and large tables. add_concurrent_foreign_key does this for us, and also checks if the foreign key already exists.

WARNING: Avoid using add_foreign_key or add_concurrent_foreign_key constraints more than once per migration file, unless the source and target tables are identical.

Data migration to fix existing records

The approach here depends on the data volume and the cleanup strategy. If we can find "invalid" records by doing a database query and the record count is not high, then the data migration can be executed in a Rails migration.

In case the data volume is higher (>1000 records), it's better to create a background migration. If unsure, contact the database team for advice.

Example for cleaning up records in the emails table in a database migration:

class RemoveRecordsWithoutUserFromEmailsTable < Gitlab::Database::Migration[2.1]
  disable_ddl_transaction!

  class Email < ActiveRecord::Base
    include EachBatch
  end

  def up
    Email.where('user_id NOT IN (SELECT id FROM users)').each_batch do |relation|
      relation.delete_all
    end
  end

  def down
    # Can be a no-op when data inconsistency is not affecting the pre and post deployment version of the application.
    # In this case we might have records in the `emails` table where the associated record in the `users` table is not there anymore.
  end
end

Validate the foreign key

Validating the foreign key scans the whole table and makes sure that each relation is correct. Fortunately, this does not lock the source table (users) while running.

NOTE: When using batched background migrations, foreign key validation should happen in the next GitLab release.

Migration file for validating the foreign key:

# frozen_string_literal: true

class ValidateForeignKeyOnEmailUsers < Gitlab::Database::Migration[2.1]
  def up
    validate_foreign_key :emails, :user_id
  end

  def down
    # Can be safely a no-op if we don't roll back the inconsistent data.
  end
end