Ecto：如何通过选择另一个连接列来预加载记录

Question

有没有办法通过选择另一个连接的列来预加载记录？

# table structure
# User 1---* Post 1---* PostTag *---1 Tag

# extract definition of scheme
scheme "posts" do
 ...
 has_many :post_tags, PostTag
 has_many :tags, [:post_tags, :tag]
end

以下伪代码表达了我的目标（但不起作用）。

query = from post in Post,
  join: user in User, on post.user_id == user.id,
  select: %{
    id: post.id,
    title: post.title,
    user_name: user.name, # <= column at joined table
  },
  preload: [:tags]
Repo.all(query)
#=> ** (Ecto.QueryError) the binding used in `from` must be selected in `select` when using `preload` in query:`

我期待这样的结果。

[
  %{id: 1, title: "AAA", user_name: "John", tags: [%{name: "elixir"},...]},
  %{id: 2, title: "BBB", user_name: "Mike", tags: [%{name: "erlang"},...]},
  ...
]

Answer 1

正如错误信息所说，你需要在预加载时选择你在from中给出的绑定，否则Ecto没有地方放置预加载的标签。这是一个简单的答案：

query = from post in Post,
  join: user in User, on: post.user_id == user.id,
  select: {post, user.name},
  preload: [:tags]

通过返回一个元组，您可以将完整的帖子和用户名放在一边。另一种方法是将帖子和用户作为完整结构返回：

query = from post in Post,
  join: user in User, on: post.user_id == user.id,
  preload: [:tags, user: user]

或者如果您不想要所有字段：

query = from post in Post,
  join: user in User, on: post.user_id == user.id,
  preload: [:tags, user: user],
  select: [:id, :title, :user_id, user: [:name]]