按数量查找

不一样,但我相信你可以通过回答这个问题来解决你的问题: Using named_scope with counts of child models

简而言之:

添加 wheels_count 列到车辆表中

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

现在,您可以按车轮数搜索车辆:

# 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])

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

所以车轮计数设置为当前计数