獲取當前用戶所發佈的所有微博客

Question

我正在構建微博克隆並構建時間表，我需要獲取當前用戶關注的任何人發佈的所有微博客。

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 

但我覺得自己像一個子選擇是有點亂，我想我更喜歡通過加入做到這一點。這裏是我想要的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 

如何將這個轉換爲ActiveRecord關聯？

此外，哪種方法通常對於此任務會更快 - 子選擇還是連接？

Answer 1

試試這個：

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