Many-to-Many-to-Many relationship with need for another specific model

问题

I have a many-to-many relationship between Supermarket, Product and Brand through the Supply- and Origin-models. I also want to store which specific Product-Brand-Combination I have in my supermarket. I thought of another model (I called it Specific_Combination where I would store :supermarket_id, :product_id and :brand_id.

class Supermarket < ActiveRecord::Base
  has_many :supplies
  has_many :products, :through => :supplies
end

class Supply < ActiveRecord::Base  
  belongs_to :product  
  belongs_to :supermarket  
end

class Product < ActiveRecord::Base
  has_many :supplies
  has_many :supermarkets, :through => :supplies

  has_many :origins
  has_many :brands, :through => :origins
end

class Origin < ActiveRecord::Base
  belongs_to :products
  belongs_to :brands
end

class Brand < ActiveRecord::Base
  has_many :origins
  has_many :products, :through => :origins
end

And now the class I thought i could use to store a specific Product-Brand-combination

class Specific_Combination < ActiveRecord::Base
  # to show which columns I would use:
  attr_accessible :supermarket_id, :product_id, :brand_id
end

Is this a suitable approach?
How do I have to model the relationships to and from Specific_Combination?
How would I access (create...) the items in Specific_Combination?
How would a better approach (normalization) look like?

Edit

class Supply < ActiveRecord::Base  
  belongs_to :origin  
  belongs_to :supermarket  
end

class Product < ActiveRecord::Base
  has_many :origins
end

class Origin < ActiveRecord::Base
  belongs_to :product
  belongs_to :brands
end

class Brand < ActiveRecord::Base
  has_many :origins
end

class Supermarket < ActiveRecord::Base
  has_many :supplies
  has_many :origins, :through => :supplies

  # my attempt to create an array of names of supermarkets
    def self.to_be_chosen
      chosen_supermarket = Array.new
      Supermarket.find_each do |supermarket|
        chosen_supermarket << supermarket.name
      end
    return chosen_supermarket
    end
end

/Edit

回答1:

Here, I might have supply belong to origin instead of product.

Also, think about whether you need SpecificCombination. What operations are you going to do with it? Are you going to list all the SpecificCombinations in your database? Are you going to search to find a specific one? Are you going to create a new combination?

Then think if these operations can be done simply with the other classes?

class Supermarket < ActiveRecord::Base
  has_many :supplies
  has_many :origins, :through => :supplies

  def self.choice
    Supermarket.all.map { |supermarket| [ supermarket.name, supermarket.id ] }
  end
end

end

class Supply < ActiveRecord::Base  
  belongs_to :origin
  belongs_to :supermarket  
end

class Origin < ActiveRecord::Base
  belongs_to :supplies
  belongs_to :brands
end


walmart = Supermarket.create(:name => "Walmart");

cornflakes = Product.create(:name => "Corn Flakes");
kellogs = Brand.create(:name => "Kellog's");
walmart.origins.create(:product_id => cornflakes, :brand_id = kellogs)

来源：https://stackoverflow.com/questions/12445675/many-to-many-to-many-relationship-with-need-for-another-specific-model

标签

ruby-on-rails

database-design

activerecord

many-to-many