ruby-on-rails – activerecord has_many：通过一次sql调用查找

class User < ActiveRecord::Base
  has_many :permissions,:dependent => :destroy
  has_many :roles,:through => :permissions
end

class Permission < ActiveRecord::Base
  belongs_to :role
  belongs_to :user
end
class Role < ActiveRecord::Base
  has_many :permissions,:dependent => :destroy
  has_many :users,:through => :permissions
end

我想在一个sql语句中找到一个用户和它的角色,但我似乎无法实现这一点：

以下声明：

user = User.find_by_id(x,:include => :roles)

给我以下问题：

User Load (1.2ms)   SELECT * FROM `users` WHERE (`users`.`id` = 1) LIMIT 1
  Permission Load (0.8ms)   SELECT `permissions`.* FROM `permissions` WHERE (`permissions`.user_id = 1) 
  Role Load (0.8ms)   SELECT * FROM `roles` WHERE (`roles`.`id` IN (2,1))

不完全理想.我如何做到这一点,以便它使用连接执行一个SQL查询并将用户的角色加载到内存中,这样说：

user.roles

不会发出新的SQL查询