module ActiveModel::Conversion

in Ruby 1.8.x responds to :id.
Note the default implementation uses persisted? just because all objects

if the object is persisted or not.
Returns an Enumerable of all key attributes if any is set, regardless

def to_key
  persisted? ? [id] : nil
end

your object with Active Model compliant methods.
define :to_model yourself returning a proxy object that wraps
If your model does not act like an Active Model object, then you should

self.
you can use the default to_model implementation, which simply returns
If your object is already designed to implement all of the Active Model

def to_model
  self
end

or nil if persisted? is false
Returns a string representing the object's key suitable for use in URLs,

def to_param
  persisted? ? to_key.join('-') : nil
end