Single Table Inheritance

Summary: don’t use Single Table Inheritance (STI), use separate tables instead.

Rails makes it possible to have multiple models stored in the same table and map these rows to the correct models using a type column. This can be used to for example store two different types of SSH keys in the same table.

While tempting to use one should avoid this at all costs for the same reasons as outlined in the document “Polymorphic Associations”.

Solution

The solution is very simple: just use a separate table for every type you’d otherwise store in the same table. For example, instead of having a keys table with type set to either Key or DeployKey you’d have two separate tables: keys and deploy_keys.

In migrations

Whenever a model is used in a migration, single table inheritance should be disabled. Due to the way Rails loads associations (even in migrations), failing to disable STI could result in loading unexpected code or associations which may cause unintended side effects or failures during upgrades.

class SomeMigration < Gitlab::Database::Migration[1.0]
  class Services < ActiveRecord::Base
    self.table_name = 'services'
    self.inheritance_column = :_type_disabled
  end

  def up
  ...

If nothing needs to be added to the model other than disabling STI or EachBatch, use the helper define_batchable_model instead of defining the class. This ensures that the migration loads the columns for the migration in isolation, and the helper disables STI by default.

class EnqueueSomeBackgroundMigration < Gitlab::Database::Migration[1.0]
  disable_ddl_transaction!

  def up
    define_batchable_model('services').select(:id).in_batches do |relation|
      jobs = relation.pluck(:id).map do |id|
        ['ExtractServicesUrl', [id]]
      end

      BackgroundMigrationWorker.bulk_perform_async(jobs)
    end
  end
  ...