为什么我无法在变更集中转换关联？

Question

我正在尝试执行替换唯一位置值的事务。当我重新插入节目时，即使我已经预装了节目并且我在变更集中使用了cast_assoc，剧集也不会附加。以下是文件：

show_controller.ex

original_show = Repo.get!(Show, id)
|> Repo.preload(:episodes)

old_position = original_show.position
%{"position" => new_position} = show_params
new_position = String.to_integer(new_position)

query = from(s in Show, where: s.position <= ^new_position, where: s.position > ^old_position)
shows = Repo.all(query) |> Repo.preload(:episodes) |> Enum.sort(&(&1.position < &2.position))

result = Repo.transaction(fn ->
  Repo.delete!(original_show)
  Repo.delete_all(query)

  Enum.each shows, fn show ->
    show = Map.put(show, :position, show.position - 1)
    changeset = Show.changeset(show)
    case Repo.insert(changeset) do
     {:ok, show} -> show
     {:error, changeset} -> Repo.rollback(changeset)
    end
  end

  changeset = Show.changeset(original_show, new_show_params)
  case Repo.insert(changeset) do
   {:ok, show} -> show
   {:error, changeset} -> Repo.rollback(changeset)
  end
end)

show.ex

 def changeset(struct, params \\ %{}) do
   struct
   |> cast(params, [:title, :position, :shuffle_episodes])
   |> cast_attachments(params, [:large_artwork_url, :parallax_artwork_url])
   |> cast_assoc(:episodes)
   |> validate_required([:title, :position, :large_artwork_url, :parallax_artwork_url, :shuffle_episodes])
   |> unique_constraint(:position)
 end