ActiveRecord的查找包含至少一个项的类别 [英] ActiveRecord find categories which contain at least one item

问题描述

支持我有两个模型的项目和种类，在许多对许多关系

Support I have two models for items and categories, in a many-to-many relation

class Item < ActiveRecord::Base
  has_and_belongs_to_many :categories 

class Category < ActiveRecord::Base
  has_and_belongs_to_many :items

现在我想筛选出其中至少包含一个项目，会是怎样做到这一点的最好办法类别？

Now I want to filter out categories which contain at least one items, what will be the best way to do this?

推荐答案

请注意，其他人answererd什么都不是高性能！

最高效的解决方案：的

更好用的 counter_cache 工作，并保存items_count模型！

better to work with a counter_cache and save the items_count in the model!

scope :with_items, where("items_count > 0")


has_and_belongs_to_many :categories, :after_add=>:update_count, :after_remove=>:update_count

def update_count(category)
  category.items_count = category.items.count
  category.save
end

对于正常belongs_to的关系你只写

for normal "belongs_to" relation you just write

belongs_to :parent, :counter_cache=>true

和在parent_model你有items_count一个字段​​（项是多元化的has_many类名）

and in the parent_model you have an field items_count (items is the pluralized has_many class name)

的http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

在has_and_belongs_to_many关系，你必须写它作为自己的如上

in a has_and_belongs_to_many relation you have to write it as your own as above

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