sql – 与ActiveRecord和Rails 3的复杂JOIN

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

我必须找到属于用户所属组的所有帖子.我用这种方法做到了：

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

但它会产生效率低下的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

我想做一个像这样的查询：

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的情况下执行此操作？

解决方法

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

（编辑：李大同）

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