Ruby on rails 在rails中查找无子级或有子级的多态关联的父级

我的RoR项目中有两个模型：汽车和图片

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

及

---

我如何才能找到所有只有儿童图片的汽车？

这可能会变得混乱/缓慢，但一个选择是迭代所有汽车，并检查是否有儿童排队

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

或者如果你想提高性能

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

---

我还没有测试过它，但我认为类似的东西可以在单个查询中实现

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

---

我自己无法测试它。。。但我希望至少它能给你一个想法。

谢谢你提供了很好的解决方案！这对我有用。我刚刚修正了一个小表达式：conditions=>[pictures.imageable_id不是NULL]我在紧急情况下考虑过这个解决方案。不过还是要谢谢你，这也行。

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

Car.with_child_picture