获取当前用户关注的人发布的所有微博答案

【问题标题】：Get all microposts posted by people the current user follows获取当前用户关注的人发布的所有微博
【发布时间】：2011-12-24 01:15:35
【问题描述】：

我正在构建一个 twitter 克隆并构建时间线，我需要获取当前用户关注的任何人发布的所有微博。

Railstutorial.org implements it like this:

class Micropost < ActiveRecord::Base
  default_scope :order => 'microposts.created_at DESC'

  # Return microposts from the users being followed by the given user.
  scope :from_users_followed_by, lambda { |user| followed_by(user) }

  private

    # Return an SQL condition for users followed by the given user.
    # We include the user's own id as well.
    def self.followed_by(user)
      following_ids = %(SELECT followed_id FROM relationships
                        WHERE follower_id = :user_id)
      where("user_id IN (#{following_ids}) OR user_id = :user_id",
            { :user_id => user })
    end
end

但我觉得子选择有点乱，我想我更喜欢通过连接来完成这个。这是我想要的 SQL：

SELECT m.*
FROM   Users u
       INNER JOIN Follows f
         ON u.id = f.follower_id
       INNER JOIN Microposts m
         ON s.user_id = f.followee_id
WHERE  u.id = [current users id]
ORDER  BY m.posted_at DESC

如何将其转换为 ActiveRecord 关联？

此外，对于此任务，哪种方法通常更快 - 子选择或连接？

【问题讨论】：

你格式化 SQL 的方式伤了我的眼睛。

标签： sql ruby-on-rails activerecord join

【解决方案1】：

试试这个：

class User
  has_many :posts

  has_many :follower_links, :class_name => "Follow", :foreign_key => :followee_id
  has_many :followers, :through => :follower_links, :source => follower

  has_many :followee_links, :class_name => "Follow", :foreign_key => :follower_id
  has_many :followees, :through => :followee_links, :source => followee

  def followee_posts
    Post.joins("JOIN (#{followees.to_sql}) AS followees 
      ON posts.user_id = followees.id")
  end
end

class Follow
  belongs_to :followee, :class_name => "User"
  belongs_to :follower, :class_name => "User"
end

class Post
  belongs_to :user
end

【讨论】：