Cast_assoc issue with many_to_many

cast_assoc doesn’t seem to work when I create or update with an existing item in the many_to_many relationship and the item has a required field. See below for example:

defmodule EctoTest.Repo.Migrations.AddTodoListsAndItems do
  use Ecto.Migration

  def change do
    create table("todo_lists") do
      add(:title, :string, null: false)
      timestamps()
    end

    create table("todo_items") do
      add(:description, :string, null: false)
      timestamps()
    end

    create table("todo_lists_items", primary_key: false) do
      add(:todo_item_id, references(:todo_items), null: true)
      add(:todo_list_id, references(:todo_lists), null: true)
    end
  end
end

defmodule EctoTest.Todos.TodoList do
  use Ecto.Schema
  alias Ecto.Changeset
  alias EctoTest.Todos.TodoItem

  schema "todo_lists" do
    field :title
    many_to_many :todo_items, TodoItem, join_through: "todo_lists_items", on_replace: :delete
    timestamps()
  end

  def changeset(struct, params \\ %{}) do
    struct
    |> Changeset.cast(params, [:title])
    |> Changeset.cast_assoc(:todo_items, required: true)
  end
end

defmodule EctoTest.Todos.TodoItem do
  use Ecto.Schema
  alias Ecto.Changeset

  schema "todo_items" do
    field :description
    timestamps()
  end

  def changeset(struct, params \\ %{}) do
    struct
    |> Changeset.cast(params, [:description])
    |> Changeset.validate_required([:description])
  end
end

defmodule EctoTest.Todos do
  alias EctoTest.Repo
  alias EctoTest.Todos.TodoItem
  alias EctoTest.Todos.TodoList

  def create_todo_item(attrs \\ %{}) do
    %TodoItem{}
    |> TodoItem.changeset(attrs)
    |> Repo.insert()
  end

  def create_todo_list(attrs \\ %{}) do
    %TodoList{}
    |> TodoList.changeset(attrs)
    |> Repo.insert()
  end

  def update_todo_list(%TodoList{} = todo_list, attrs \\ %{}) do
    todo_list
    |> Repo.preload([:todo_items])
    |> TodoList.changeset(attrs)
    |> Repo.update()
  end
end

Both the following tests fail with similar errors

defmodule EctoTest.Todos.TodoListTest do
  use EctoTest.DataCase
  alias EctoTest.Todos

  describe "create_todo_list/2" do
    test "creates todo_list and casts todo_items" do
      {:ok, todo_item} = Todos.create_todo_item(%{description: "Test description"})
      todo_list_params = %{title: "Test title", todo_items: [%{id: todo_item.id}]}

      assert {:ok, todo_list} = Todos.create_todo_list(todo_list_params)
    end
  end

  describe "update_todo_list/2" do
    test "updates todo_list and casts todo_items" do
      {:ok, todo_item} = Todos.create_todo_item(%{description: "Test description"})
      todo_list_params = %{title: "Test title", todo_items: [%{id: todo_item.id}]}

      assert {:ok, todo_list} = Todos.create_todo_list(todo_list_params)

      {:ok, new_todo_item} = Todos.create_todo_item(%{description: "New todo item"})
      todo_list_params = %{todo_items: [%{id: new_todo_item.id}]}

      assert {:ok, todo_list} = Todos.update_todo_list(todo_list, todo_list_params)
    end
  end
end

These are the errors I get

1) test update_todo_list/2 updates todo_list and casts todo_items (EctoTest.Todos.TodoListTest)
     test/ecto_test/todos/todo_list_test.exs:15
     match (=) failed
     code:  assert {:ok, todo_list} = Todos.create_todo_list(todo_list_params)
     right: {:error,
             #Ecto.Changeset<
               action: :insert,
               changes: %{
                 title: "Test title",
                 todo_items: [
                   #Ecto.Changeset<
                     action: :insert,
                     changes: %{},
                     errors: [
                       description: {"can't be blank", [validation: :required]}
                     ],
                     data: #EctoTest.Todos.TodoItem<>,
                     valid?: false
                   >
                 ]
               },
               errors: [],
               data: #EctoTest.Todos.TodoList<>,
               valid?: false
             >}
     stacktrace:
       test/ecto_test/todos/todo_list_test.exs:19: (test)



  2) test create_todo_list/2 creates todo_list and casts todo_items (EctoTest.Todos.TodoListTest)
     test/ecto_test/todos/todo_list_test.exs:6
     match (=) failed
     code:  assert {:ok, todo_list} = Todos.create_todo_list(todo_list_params)
     right: {:error,
             #Ecto.Changeset<
               action: :insert,
               changes: %{
                 title: "Test title",
                 todo_items: [
                   #Ecto.Changeset<
                     action: :insert,
                     changes: %{},
                     errors: [
                       description: {"can't be blank", [validation: :required]}
                     ],
                     data: #EctoTest.Todos.TodoItem<>,
                     valid?: false
                   >
                 ]
               },
               errors: [],
               data: #EctoTest.Todos.TodoList<>,
               valid?: false
             >}
     stacktrace:
       test/ecto_test/todos/todo_list_test.exs:10: (test)

Is this expected behavior? If so, what’s the best way for me to make this work?