这就是模型的样子：

class Car < ActiveRecord::Base  
  has_many :car_colors
  has_many :colors,:through => :car_colors
end

class CarColor < ActiveRecord::Base
  belongs_to :color
  belongs_to :car
end

class Color < ActiveRecord::Base  
  has_many :car_colors
  has_many :cars,:through => :car_colors
end

这是查询：

@cars = Car.all(:joins => :car_colors,:conditions => { :car_colors => {:color_id => params[:id_number]}},:order => "cars.created_at DESC")

并输出错误：

PG::Error: ERROR:  column reference "created_at" is ambiguous
LINE 1: ...d" WHERE "car_colors"."color_id" = 2 AND (created_at...
                                                             ^
: SELECT "cars".* FROM "cars" INNER JOIN "car_colors" ON "car_colors"."car_id" = "cars"."id" WHERE "car_colors"."color_id" = 2 AND (created_at > '2013-05-03 12:28:36.551058') ORDER BY cars.created_at DESC

生成的SQL查询(错误消息下面)似乎很好,但是什么原因导致错误消息？

先谢谢你.