What's a good design for an intermediate triple-relation model?

Question

Hello,

I'm new to Rails and don't grasp yet all the possibilities with associations. Here's my problem:

I have a couple of models like apple and lemon. Then there's the model 'relation' that holds triples of relations:

subject | relation | object

apple | is sweeter than | lemon

The migration for 'relations' is this:

create_table :relations do |t|
  t.references :subject,  :polymorphic => true
  t.string     :relation
  t.references :object,   :polymorphic => true
  t.timestamps    
end

this should store relations like

subject_id = 1

subject_type = apple

relation = is sweeter than

object_id = 2

object_type = lemon

In reality I have more than just 2 models, so I thought I need to make the subject and object column model-agnostic by using the polymorphic option.

How would you set the associations in the model classes of apple, lemon and relation? Is the table design of relations good like this?

Thanks so much for your help!!

-Alex

Answer 1

Give the db schema you described, seems like it should be fairly straightforward:

class Relation < ActiveRecord::Base
  belongs_to :object, :polymorphic => true
  belongs_to :subject, :polymorphic => true
end

And your other classes would look like this

class Apple < ActiveRecord::Base
  has_many :object_relations, :class_name => 'Relation', :as => :object
  has_many :subject_relations, :class_name => 'Relation', :as => :subject
end

class Orange < ActiveRecord::Base
  has_many :object_relations, :class_name => 'Relation', :as => :object
  has_many :subject_relations, :class_name => 'Relation', :as => :subject
end

Answer 2

Polymorphic IS PAIN:

Unless you need it badly, use somethimng like SingleTableInheritance:

class Fruit < ActiveRecord::Base
   has_many :relations
   has_many :related_objects, :through => :relation, :class_name => 'Fruit'
   has_many :relating_subjects, :through => :relation, :class_name => 'Relation'
end
class Relation < ActiveRecord::Base
   belongs_to :object, :class => 'Fruit'
   belongs_to :subject, , :class => 'Fruit'
   validate_presence_of :object_id
   validate_presence_of :subject_id
   validate_presence_of :relation
end

And then like:

class Apple < Fruit
   ...
end

I hope that helps, (I did not tested this code)