Adding DELETE CASCADE via Ecto Migrations

Say you have a relationship in your PG database that doesn’t have cascade deletion, and you want to add it. This is how:

defmodule MyApp.Repo.Migrations.CascadeDelete do
  use Ecto.Migration

  def change do
    drop constraint(:emails, :emails_timeline_event_id_fkey)

    alter table(:emails) do
      modify :timeline_event_id, references(:timeline_events, type: :uuid, on_delete: :delete_all)
    end
  end
end

You simply have to drop the foreign key constraint first, and then re-create it with the CASCADE DELETE option.