defmodule App.Membership do
  # ...
  def changeset(struct, params \\ %{}) do
    struct
    # ...
    |> put_role
    |> cast_assoc(:user, required: true)
  end
  
  defp put_role(changeset) do
    case changeset do
      %Ecto.Changeset{valid?: true} ->
        put_change(changeset, :role, :admin)
      _ ->
        changeset
    end
  end
end