ruby-on-rails - has_many 关联的总和

Question

对 Ruby on Rails 并不陌生，但从未真正处理过更复杂的 ActiveRecord 查询。

假设我有一个会员模型，它有_many 推荐的用户和推荐的用户有_many 购买的产品。

我想要做的是一种有效的 ActiveRecord 方法来获取所有推荐用户的购买产品总数。我该怎么做呢？

谢谢。

Answer 1

假设对象如下：

class Affiliate < ActiveRecord::Base
  has_many :users
end

class Users < ActiveRecord::Base
  #should have purchased_products_count integer column
  belongs_to :affiliate
  has_many :pruchased_products
end

class PurchasedProducts < ActiveRecord::Base
  belongs_to :user, counter_cache: :purchased_products_count
end

products_count = User.first.purchased_products.size # uses counter_cache to get the size
another_products_count = User.first.purchased_products_count # get the value diretly
all_users_products_count = my_affiliate.users.map(&:purchased_products_count).inject(:+) # makes an array of product counts then sums them

我认为这也可能有效

my_affiliate.users.sum('purchased_products_count')