如何在使用preload时检查未定义的值，并在Ecto中不返回值

# 假设有两个schema：User和Post
defmodule MyApp.User do
  use Ecto.Schema
  import Ecto.Changeset

  schema "users" do
    field :name, :string
    many_to_many :posts, MyApp.Post, join_through: "user_posts"

    timestamps()
  end
end

defmodule MyApp.Post do
  use Ecto.Schema
  import Ecto.Changeset

  schema "posts" do
    field :title, :string
    many_to_many :users, MyApp.User, join_through: "user_posts"

    timestamps()
  end
end

# 查询所有用户及其关联的帖子，如果帖子不存在则不返回任何值
query = from u in MyApp.User,
        left_join: p in assoc(u, :posts),
        select: {u, p},
        where: is_nil(p.id)

# 执行查询
result = MyApp.Repo.all(query)

# 处理结果
result
|> Enum.map(fn {user, post} ->
  if post do
    %{user: user, post: post}
  else
    nil
  end
end)
|> Enum.filter(& &1 != nil)