使用Ecto v2.2.6，Phoenix 1.3 
 
 我有一个带有新闻源的博客应用。 它是这样工作的： 
 
 
 
 用户可以提交帖子。 
 
 
 用户可以关注其他用户。 
 
 
 当用户提交帖子时，会添加新闻源表格中的项目。 
 
 
 用户可以看到他们所关注的用户提交的帖子的新闻源。 
 
 
 我想使用Ecto.Query来获取给定用户所关注的用户的新闻源项目列表。 
 
 
 快速背景。 这是对象： 
 
 用户 
 
mix phx.gen.json Accounts User users email:string password:string
 
 岗位 
 
mix phx.gen.json Content Post posts title:string content:string user_id:references:users
 
 （ users和posts有一个has_many:和belongs_to: relationship。） 
 
 跟随 
 
mix phx.gen.json Accounts Follow follows following_id:references:users followed_id:references:users
 
 （当用户A跟随用户B时，会创建一个新的Follow条目，其中following_id指向A，并且followed_id指向B.） 
 
 新闻源 
 
mix phx.gen.json Content Newsfeeditem newsfeeditems type:string, user_id:integer, content:string
 
 
 现在我想查询这些东西。 为了让我能够获得给定用户的Newsfeeditems列表，这很简单： 
 
 导入Ecto.Query 
 
query =
  from n in Newsfeeditem,
    where: n.user_id == ^user_id
 
 假设我是用户1，我正在关注用户2,3和4. follows表中有三个条目。 要为这些用户获取所有相应的newsfeeditems，查询将如下所示： 
 
query =
  from n in Newsfeeditem,
    where: n.user1_id in [2,3,4]
 
 我想让它充满活力。 这是我迷路的地方。 我想做一些类似于此的事情： 
 
subquery =
  from f in Follow,
    where: f.following_id == 1,
    select: f.follower_id

query =
  from n in Newsfeeditem,
    where: n.user_id in (Repo.all(subquery))
 
 显然这不起作用，但我不确定如何正确地构造这些东西。 
 
 如何通过子查询选择此项？ （ 注意我正在寻找特定的子查询解决方案，但如果有更好的方法则可以获得奖励积分） 

Using Ecto v2.2.6, Phoenix 1.3
 
I have a Blog app with a Newsfeed. It works like this:
 
 
 
Users can submit Posts. 
 
 
Users can follow other Users. 
 
 
When a user submits a Post, an item in the Newsfeed table is added. 
 
 
Users can see a newsfeed of Posts that have been submitted by Users they are following.
 
 
I want to use Ecto.Query to get a list of newsfeed items from users that a given user is following.
 
 
Quick background. Here are the objects:
 
User
 
mix phx.gen.json Accounts User users email:string password:string
 
Post
 
mix phx.gen.json Content Post posts title:string content:string user_id:references:users
 
(users and posts have a has_many: and belongs_to: relationship.)
 
Follow
 
mix phx.gen.json Accounts Follow follows following_id:references:users followed_id:references:users
 
(When user A follows user B, a new Follow entry is created where following_id points to A, and followed_id points to B.)
 
Newsfeed
 
mix phx.gen.json Content Newsfeeditem newsfeeditems type:string, user_id:integer, content:string
 
 
Now I want to query this stuff. For me to get a list of Newsfeeditems for a given user, it's easy:
 
import Ecto.Query
 
query =
  from n in Newsfeeditem,
    where: n.user_id == ^user_id
 
Let's say I'm user 1, and I am following users 2, 3 and 4. There are three entries in the follows table. To get all the corresponding newsfeeditems for those users, the query would look something like this:
 
query =
  from n in Newsfeeditem,
    where: n.user1_id in [2,3,4]
 
I want to make it dynamic. Here's where I am lost. I would like to do something that resembles this:
 
subquery =
  from f in Follow,
    where: f.following_id == 1,
    select: f.follower_id

query =
  from n in Newsfeeditem,
    where: n.user_id in (Repo.all(subquery))
 
Obviously this doesn't work, but I'm not sure how to structure these things correctly. 
 
How can I select this via subquery? (Note I'm looking for the subquery solution specifically, but bonus points if there is a better way)

原文：https://stackoverflow.com/questions/46719696