Question

我正在运行rails 4.我们有一个User模型has_many Posts。我想创建一个“提要”，显示网站中每个用户最多三个帖子。

我能得到的最接近的是

  # GET /users
  # GET /users.json
  def index
    users = User.includes(:posts)
    users.each do |user|
       @feed_items << user.posts.limit(3)
    end
    @comment = Comment.new
  end

我的视图会显示@feed_items中的所有项目，但rails不允许你连接查询？如何在不执行n + 1查询的情况下在Rails中执行此操作？我需要自定义SQL查询吗？

参考：http://guides.rubyonrails.org/active_record_querying.html#eager-loading-associations

Answer 1

includes已经在一个查询中读取了每个用户的所有帖子。因此，您应该能够在ruby中获取前三个，而无需再次查询数据库。

users = User.includes(:posts)
@feed_items = []
users.each do |user|
   @feed_items += user.posts.first(3)
end

这构成了一长串的帖子。如果您需要每个用户列表的列表，请使用<<代替+=。

我能够测试这个。以下是生成的查询：

User Load (4.4ms)  SELECT "users".* FROM "users" 
Post Load (2.4ms)  SELECT "posts".* FROM "posts" WHERE "posts"."user_id" IN (4, 5, 6, 7, 8, 9, 10, 11, 12, 13)

结果如下：

irb(main):037:0> @feed_items.map &:subject
=> ["User 0's Post 1", "User 0's Post 2", "User 0's Post 3", "User 1's Post 1", 
"User 1's Post 2", "User 1's Post 3", "User 2's Post 1", "User 2's Post 2", 
"User 2's Post 3", "User 3's Post 1", "User 3's Post 2", "User 3's Post 3", 
"User 4's Post 1", "User 4's Post 2", "User 4's Post 3", "User 5's Post 1", 
"User 5's Post 2", "User 5's Post 3", "User 6's Post 1", "User 6's Post 2", 
"User 6's Post 3", "User 7's Post 1", "User 7's Post 2", "User 7's Post 3", 
"User 8's Post 1", "User 8's Post 2", "User 8's Post 3", "User 9's Post 1", 
"User 9's Post 2", "User 9's Post 3"]

includes的缺点是总是，您最终可能会检索不需要的数据。在这里，如果用户有超过3个帖子，那么全部撤销并丢弃除3之外的所有帖子可能比每个用户一个查询更昂贵。

服务器端解决方案需要花哨的SQL。您可以搜索“组内的sql限制”以查看一些提案。

有效地遍历所有用户并返回3个相关项[SQL] [Rails]

1 个答案: