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ActiveRecord Association Extensions

您所遇到的问题是,Rails将只使用连接表中的foreign_keys来加载您需要的关联数据.除非你真的直接加载连接模型,否则它不会给你访问连接属性的能力

一些搜索引导我们进入ActiveRecord关联扩展 – 一种访问不同ActiveRecord关联之间的中间数据的方法(使用一个名为proxy_association的集合).这将允许您从连接模型访问额外的属性,将它们附加到“原始”模型中：

#app/models/ingredient.rb
class Ingredient < ActiveRecord::Base
   attr_accessor :amount #-> need a setter/getter
end

#app/models/meal.rb
class Meal < ActiveRecord::Base
   has_many :meal_ingredients
   has_many :ingredients,through: :meal_ingredients,extend: IngredientAmount
end

#app/models/concerns/ingerdient_amount.rb
module IngredientAmount

    #Load
    def load
        amounts.each do |amount|
            proxy_association.target << amount
        end
    end

    #Private
    private

    #Amounts
    def amounts
        return_array = []
        through_collection.each_with_index do |through,i|
            associate = through.send(reflection_name)
            associate.assign_attributes({amount: items[i]}) if items[i].present?
            return_array.concat Array.new(1).fill( associate )
        end
        return_array
    end

    #######################
    #      Variables      #
    #######################

    #Association
    def reflection_name
        proxy_association.source_reflection.name
    end

    #Foreign Key
    def through_source_key
        proxy_association.reflection.source_reflection.foreign_key
    end

    #Primary Key
    def through_primary_key
        proxy_association.reflection.through_reflection.active_record_primary_key
    end

    #Through Name
    def through_name
        proxy_association.reflection.through_reflection.name
    end

    #Through
    def through_collection
        proxy_association.owner.send through_name
    end

    #Captions
    def items
        through_collection.map(&:amount)
    end

    #Target
    def target_collection
        #load_target
        proxy_association.target
    end

end

这应该将amount属性添加到您的成分对象,让您执行：

@meal = Meal.find 1
@meal.ingredients.each do |ingredient|
   ingredient.amount
end