assert_difference "List.count" do
    assert_difference "ListItem.count" do
        post list_path, params: { list: {
                                  name: "List",
                                  list_item_attributes: {
                                  list_item: { user_item_id: user_items(:milk).id,
                                               list_id: lists(:one).id,
                                               quantity: 3 } } } }
    end
end