Question

Here is an example of the structure I’m trying to achieve. I’ll just setup a silly example so it’s easier to explain:

Let’s say we have a Product that holds a bunch of different hammers. Each hammer has multiple attributes (weight, size, color, etc) but they all can hammer nails in. So in that respect they are interchangeable. However they are not interchangeable with other products, like a chainsaw or a sledgehammer.

So I want to be able to keep a list of all products that are interchangeable. So if one hammer is not available I could see ids all the other products that I can give to a customer instead.

Since I don’t know how many interchangeable products each product can have ( let’s say there are 5 different hammers, and 50 screwdrivers), I can’t just create an interchangeable field to keep this info. i was thinking about enums, but they are more complicated to run reports on.

Here is what I have so far, but I’m not sure if this is the best solution (it’s kind of late and my mind is starting to melt – this is a much simplified example if my self referential tables):

class Product < ActiveRecord::Base

  has_many :interchangable_products, :dependent => :destroy
end


class InterchangableProduct < ActiveRecord::Base
  belongs_to :product, :class => "Product", :foreign_key => :product_id


  belongs_to :interchangable_with, :class_name => "InterchangableProduct", :foreign_key => :interchangable_with_id
  has_many   :interchangables, :class_name => "InterchangableProduct",  :inverse_of => :interchangable_with, :foreign_key => :interchangable_with_id

  validates :product_id, :presence => true, :uniqueness => [:scope => :interchangable_with_id]
end

Thank you

Answer 1

Thanks to a comment from Dave Newton I arrived to a solution that looks sort of like this:

class InterchangeableProductRelationship < ActiveRecord::Base
  belongs_to :product, :class_name => "Product", :foreign_key => "product_id"
  belongs_to :interchangeable_product, :class_name => "Product", :foreign_key => "interchangeable_product_id"

  validates :product_id, :presence => true, :uniqueness => [:scope => :interchangeable_product_id]
end


class Product < ActiveRecord::Base
  has_many :relations_to, :foreign_key => 'product_id', :class_name => "InterchangeableProductRelationship"
  has_many :relations_from, :foreign_key => 'interchangeable_product_id', :class_name => "InterchangeableProductRelationship"  
  has_many :linked_to, :through => :relations_to, :source => :interchangeable_product
  has_many :linked_from, :through => :relations_from, :source => :product


  def interchanges_with
    self.linked_to | self.linked_from
  end 
end

Hopefully this will be helpful to someone.