Policy for explicitly setting relationship on create

  defmacro is_a(attr, type, opts \\ []) do
    quote do
      relationships do
        belongs_to unquote(attr), unquote(type) do
          source_attribute unquote(opts[:source_attribute] || :id)
          primary_key? true
          allow_nil? false
        end
      end

      # this part is not in yet
      changes do
        change MaybeCreateParent, on: [:create]
        # TODO
      end
    end
  end

actions do
  create :create do
    argument :parent, App.Parent
    # ...
  end
end

defmodule MaybeCreateParent do
  def change(cs, _, _) do
    parent = Changeset.get_argument(cs, :parent) || %{}

    cs
    |> Changeset.delete_argument(:parent)
    |> Changeset.manage_relationship(:parent, parent, type: :create)
  end
end

  policies do
    policy [action_type(:create), changing_relationship(:parent)] do
      authorize_if IsSomeRole
      authorize_if IsAnotherRole
      deny_unless never()
    end
  end