Question

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

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

Answer 1

please notice, what the other guys answererd is NOT performant!

the most performant solution:

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

for normal “belongs_to” relation you just write

belongs_to :parent, :counter_cache=>true

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

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

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