ruby-on-rails - Rails 通过关系使用 has_many 查找和计算

Question

我有以下型号：

产品：

class Product < ActiveRecord::Base
  has_many :product_recommendations, :dependent => :destroy
  has_many :recommendations, :through => :product_recommendations  
end

产品推荐：

class ProductRecommendation < ActiveRecord::Base
  belongs_to :recommendation
  belongs_to :product
end

推荐：

class Recommendation < ActiveRecord::Base
  has_many :product_recommendations, :dependent => :destroy
  has_many :products, :through => :product_recommendations

  has_many :recommendation_ratings, :dependent => :destroy
  has_many :ratings, :through => :recommendation_ratings
end

评分：

class Rating < ActiveRecord::Base
  has_many :recommendation_ratings, :dependent => :destroy
  has_many :recommendations, :through => :recommendation_ratings
end

推荐评分：

class RecommendationRating < ActiveRecord::Base
  belongs_to :recommendation
  belongs_to :rating  
end

我如何能够计算给定推荐的平均评分？我的评级表仅包含 4 条记录（评级 1-4），并且我的推荐控制器中有一个操作，它更新 RecommendationsRatings 连接表以将评级与推荐相关联。谢谢！

Answer 1

你有没有尝试过：

class Product < ActiveRecord::Base
  has_many :product_recommendations, :dependent => :destroy
  has_many :recommendations, :through => :product_recommendations
  has_many :ratings, :through => :recommendations
end

接着

p = Product.find 1
p.ratings.average(:rating)  

这是假设 Ratings 模型有一个名为 rating 的 int / float 存储等级