ActiveRecord self-referential many-to-many relationships with extra attributes

20110624183946_create_example_tables.rb

class CreateExampleTables < ActiveRecord::Migration
  def self.up
    create_table :masters do |t|
      t.string :fname
      t.string :lname

      t.timestamps
    end

    create_table :master_relations do |t|
      t.integer :master_id, :null => false
      t.integer :relation_id, :null => false
      t.string :relation_type, :null => false

      t.timestamps
    end
  end

  def self.down
    drop_table :masters
    drop_table :master_relations
  end
end

example.rb

# The relationship names could be improved, but this works
# Not sure if it is the most elegant way, but it should allow you to
# store and query relationship types

bt = Master.create(:fname => 'Brandon', :lname => 'Turner')
mt = Master.create(:fname => 'Marcia', :lname => 'Turner')

bt.master_relations.create(:relation => mt, :relation_type => 'Wife')
mt.master_relations.create(:relation => bt, :relation_type => 'Husband')

# All of Brandon's relations, regardless of type
bt.relations
  => [#<Master id: 2, fname: "Marcia", lname: "Turner"]

# Brandon's wives?
bt.relations_by('Wife')
  => [#<Master id: 2, fname: "Marcia", lname: "Turner"]

# Brandon's husbands?
bt.relations_by('Husband')
  => []

# Who is Brandon a husband to?
bt.inverse_relations_by('Husband')
  => [#<Master id: 2, fname: "Marcia", lname: "Turner"]

master.rb

class Master < ActiveRecord::Base
  has_many :master_relations
  has_many :inverse_master_relations, :class_name => 'MasterRelation', :foreign_key => :relation_id

  has_many :relations, :through => :master_relations
  has_many :inverse_relations, :through => :inverse_master_relations, :source => :master

  def relations_by(relation_type)
    relations.where(:master_relations => {:relation_type => relation_type})
  end

  def inverse_relations_by(relation_type)
    inverse_relations.where(:master_relations => {:relation_type => relation_type})
  end
end

master_relation.rb

class MasterRelation < ActiveRecord::Base
  belongs_to :master
  belongs_to :relation, :class_name => 'Master'
end