Composite Primary Keys using Elixir Ecto

Ecto allows us to map a table with composite primary keys into our schemas. The way to do that is by using the primary_key: true option for the Ecto.Schema.field/3 instead of dealing with the special module attr @primary_key. Check this out:

defmodule MyApp.OrderItem do
  use Ecto.Schema

  @primary_key false
  schema "order_items" do
    field :product_id, :integer, primary_key: true
    field :order_id, :integer, primary_key: true
    field :quantity, :integer
  end
end