Question

假设我有一个模型

Vehicle

和它

has_many :wheels

什么是正确的发现/：conditions =＆gt; ...声明只找到有3个或更多轮子的车辆，例如：三轮车，汽车，卡车，坦克等......？

Answer 1

不一样，但我相信你可以借助这个问题的答案来解决你的问题：Using named_scope with counts of child models

简而言之：

将wheels_count列添加到您的车辆表

class Wheel < ActiveRecord::Base
  belongs_to :vehicle, :counter_cache => true # this will autoupdate the wheels_count on Vehicle
end

现在您可以按车轮数量搜索您的车辆：

# for all Vehicles with 4 Wheels
Vehicle.all(:condition => { :wheels_count => 4 })
# or for all Vehicles with more than 3 Wheels
Vehicle.all(:condition => ["wheels_count > ?", 3])

<强>更新

class AddWheelCount < ActiveRecord::Migration
  def self.up  
    add_column :vehicles, :wheels_count, :integer, :default => 0  

    Vehicle.reset_column_information  
    Vehicle.all.each do |p|  
      p.update_attribute :wheels_count, p.wheels.length  
    end  
  end  

  def self.down  
    remove_column :vehicles, :wheels_count  
  end
end

所以wheels_count设置为当前计数

按数量查找

1 个答案: