specification :user do
  should_require_presence_of :username
end

specification :authenticated_user do
  should_satisfy :user_specification
  should_require_presence_of :password
  should_require_confirmation_of :password
end

ActiveRecord::Base.send(:include, ActiveSpec::Satisfies)

class User < ActiveRecord::Base
  should_satisfy :user_specification
  should_satisfy :authenticated_user_specification, :if => :authorised?
end

u = User.new
u.satisfies_specs? # returns false
u.username = 'joebloggs'
u.satisfies_specs? # returns true
u.authorised = true
u.satisfies_specs? # returns false
u.password = 'foobar'
u.satisifies_specs? # returns true