ecto.ex

  def insert_built_belongs_to_assoc(module, foreign_key, association_name, record) do
    case Map.get(record, association_name) do
      built_relation = %{__meta__: %{state: :built}} ->
        relation = built_relation |> module.insert
        relation_pkey_value = relation |> get_association_pkey_value
        Map.put(record, foreign_key, relation_pkey_value)
      _ -> Map.delete(record, foreign_key)
    end
  end

  defp get_association_pkey_value(association) do
    [primary_key] = association.__struct__.__schema__(:primary_key)
    %{^primary_key => value} = association
    value
  end