带有ActiveRecord和Rails 3的复杂JOIN [英] Complex JOIN with ActiveRecord and Rails 3

问题描述

我有以下模型：

class User < ActiveRecord::Base
  has_many :memberships
  has_many :groups, :through => :memberships
end

class Group < ActiveRecord::Base
  has_many :memberships
  has_many :users, :through => :memberships
end

class Membership < ActiveRecord::Base
  belongs_to :user
  belongs_to :group
end

class Post < ActiveRecord::Base
  belongs_to :group
end

我必须查找所有属于user是成员的组的帖子。我已经用这种方法做到了：

I have to find all posts that belong to groups where user is a member. I have made it with this method:

@post = Post
  .joins(:group => {:memberships => :user})
  .where(:memberships => {:user_id => current_user.id})

但它会产生无效的SQL：

but it produces unefficient SQL:

SELECT "posts".* FROM "posts" 
INNER JOIN "groups" ON "groups"."id" = "posts"."group_id" 
INNER JOIN "memberships" ON "memberships"."group_id" = "groups"."id" 
INNER JOIN "users" ON "users"."id" = "memberships"."user_id" 
WHERE "memberships"."user_id" = 1

我想这样查询：

SELECT posts.* FROM posts 
INNER JOIN memberships ON memberships.group_id = posts.group_id 
WHERE memberships.user_id = 1

如何在不使用原始SQL的情况下做到这一点？

How can I do this without using raw SQL?

推荐答案

类似这样的东西为您工作，尽管它需要混入一些原始SQL

something like this should work for you, although it requires mixing in a little raw SQL

Post
  .joins("INNER JOIN memberships ON memberships.group_id = posts.group_id")
  .where(:memberships => {:user_id => current_user.id})

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