ruby-on-rails - 在 Rails 中的多态关联中查找没有或有孩子的父母

Question

我有两个模型：我的 RoR 项目中的汽车和图片

class Car < ActiveRecord::Base
  has_many :pictures, :as => :imageable, :dependent => :destroy
end

和

class Picture < ActiveRecord::Base
  belongs_to :imageable, :polymorphic => true, :dependent => :destroy
end

我如何才能找到所有只有儿童照片的汽车？

Answer 1

我还没有测试过它，但我认为这样的事情会在一个查询中完成。

class Car < ActiveRecord::Base
    has_many :pictures, :as => :imageable, :dependent => :destroy
    named_scope :with_child_picture, lambda { { 
        :joins => :pictures, 
        :group => "pictures.imageable_id", 
        :conditions => ["pictures.imageable_id != ?", nil] 
    } }
end

你可以用它作为

Car.with_child_picture

我自己无法测试它......但我希望它至少能给你一个想法。

Answer 2

这可能会变得混乱/缓慢，但一种选择是遍历所有汽车并检查孩子的数量是否排队。

good_cars = []
Cars.all.each do |car|
  if (car.pictures.count > 0 && car.pictures.count == Picture.find_by_car_id(car.id).count)
    good_cars << car
  end
end

或者，如果您想要提高性能

good_cars = []
Cars.all.each do |car|
 if (car.pictures.count > 0)
    pic_count_for_car = Picture.count_by_sql(
      "SELECT COUNT(*) from pictures WHERE car_id = #{car.id}"
    )
    if (car.pictures.count == pic_count_for_car) 
      good_cars << car
    end
  end
end