使用 ActiveRecord 和 Rails 3 进行复杂 JOIN

我有以下型号：

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。我用这个方法做到了：

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

但它会产生低效的 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

我想进行这样的查询：

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

在不使用原始 SQL 的情况下如何做到这一点？

通过从调用中删除未使用的连接，您可以在不更改模型的情况下更接近：

Post.joins(group: :memberships).where(memberships: { user_id: 1 })

编译为 SQL

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