class User
    attr_accessor :id, :name
  end

  require 'factory_girl'

  Factory.define :user, :class => User do |user|
    user.name 'John Doe'
    user.after_build do |object|
      object.id = 'john.doe' # it might be pointless to assign it here, but this is just an example.
    end
  end

  Factory.define :child_user, :parent => :user do |user|
    user.name 'John Doe Jr.'
    user.after_build do |object|
      object.id = 'john.doe.jr'
    end
  end

  child = Factory.build(:child_user)
  puts child.name # => "John Doe Jr." which is fine.
  puts child.id   # => "john.doe" which is different from what I expect.