cheat sheets.

Single Table Inheritance

class CreateSTI < ActiveRecord::Migration
  def self.up
    create_table "statuses" do |t|
      t.column :type, :string # You need this!
      t.column :invoice_id, :integer
      t.column :employee_id, :integer
      t.column :created_at, :datetime
    end
  end

  def self.down
    drop_table "statuses"
  end
end

class Status < ActiveRecord::Base
  #superclass
end

class Invoiced < Status
  #subclass
  belongs_to :invoice
end

class Employeed < Status
  #subclass
  belongs_to :employee, :class_name => "Person", :foreign_key => 'employee_id'
end

Fixtures for a STI model must go in the fixture file for the
superclass.

To find out what class an instance of the superclass use
@instance.class or you can use the value of the type column
from other systems that do not go through AR.

If you cannot use type (radio button helper has problems
with it) define the new column in your superclass model
like this:
def self.inheritance_column
 'role' 
end