address.rb

class Address < ActiveRecord::Base
  belongs_to :suburb
  belongs_to :thing, :polymorphic => true
end

Fields:
  thing_id
  thing_type
  suburb_id

client.rb

class Client < ActiveRecord::Base
  has_one :address, :as => :thing, :dependent => :destroy
end

Fields:
  name

employer.rb

class Employer < ActiveRecord::Base
  has_one :address, :as => :thing, :dependent => :destroy
end

Fields:
  name

problem

Finding a suburb with a postcode of "5114", and getting all the employers.

suburb.rb

class Suburb < ActiveRecord::Base
  has_many :addresses
end

Fields:
  id
  name
  postcode

Yuji Yokoo's Solution

SQL statement:

SELECT employers.* FROM `suburbs`, `addresses`, `employers` WHERE suburbs.postcode = "5114" AND suburbs.id = addresses.suburb_id AND addresses.thing_id = employers.id and addresses.thing_type = 'employer';

Oops, could call this with #find_by_sql and it does exactly as it should. Yuji's the man.