Question

我正在建立一个推特克隆并构建我需要的时间线，以便当前用户随后发布所有微博。

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

但是我觉得副选择有点乱，我想我更喜欢通过连接完成这个。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

如何将此转换为ActiveRecord关联？

此外，对于此任务，哪种方法通常会更快 - 子选择或连接？

Answer 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

获取当前用户关注的人发布的所有微博

1 个答案: