Question

Let’s say I have a person table, and I want it to classify all people as either alphas or omegas. All omegas have exactly one alpha but no omegas, all alphas have any number of omegas but no alpha.

This is a simple two level hierarchy, which I can encode using a single foreign key:

CREATE TABLE people (
  id INTEGER NOT NULL PRIMARY KEY,
  alpha_id INTEGER FOREIGN KEY REFERENCES people, 
  -- alpha_id is NULL if and only if this person is an alpha
  -- other stuff we know about people...
);

Now I can create a general person class, but it gets slightly awkward when I get to the alpha-omega relationship.

class Person < ActiveRecord::Base
  # ... stuff I know about people

  # if alpha_id is NULL
    has_many :omegas, :as => :alpha, :class_name => Person
  # else 
    belongs_to :alpha, :class_name => Person
end

It’d be nice to split the person off into two subclasses, one for Alphas, one for Omegas, but I’m not sure how well that’d play with ActiveRecord.

Ideally, I’d like something like this:

class Person < ActiveRecord::Base
  # ... stuff I know about people
end
class Alpha < Subset(Person)
  column_is_null :alpha_id
  has_many :omegas
end
class Omega < Subset(Person)
  column_is_not_null :alpha_id
  belongs_to :alpha
end

Is this sort of subclassing, or something approximating it, available in ActiveRecord?

Answer 1

Use named_scope:

 class Person < ActiveRecord::Base
   # ... stuff I know about people
   named_scope :alphas, :conditions => { :alpha_id => nil }
   named_scope :omegas, :conditions => "alpha_id IS NOT NULL"

   # if alpha_id is NULL
   has_many :omegas, :as => :alpha, :class_name => Person
   # else 
   belongs_to :alpha, :class_name => Person
 end

Now you can reference Person.alphas and Person.omegas to get the objects you are looking for. Does this help?