ImmutableAttributes
===================

When you want to prevent certain attributes from being changed once set you can declare them as immutable:

class MyModel < ActiveRecord::Base
  attr_immutable :permalink, :param_identifier
end

When MyModel.find(:first).permalink = 'anything' is called it will raise an ImmutableAttributeError
MyModel.new.permalink = 'works!' will properly set the value because the record is unsaved.

If you'd only like this to happen for certain conditions, and want to handle other attribute-writers, too (like 
update_attribute), you can use the validation.

validates_immutable :permalink

Configuration options for the validation:
* :message - A customer error message (default is: "can't be changed")
* :if - Specifies a method, proc, or string to call to determine if the validation should occure (e.g., :if => 
:allow_validation or :if => Proc.new{|user| user.signup_step > 2}. The method, proc or string should return or evaluate 
to a true or false value.
* :unless - Specifies a method, proc, or string to call to determine if the validation should not occure (e.g., :unless 
=> :skip_validation or :unless => Proc.new{|user| user.signup_step <= 2}. The method, proc or string should return or 
evaluate to a true or false value.

Created by Jack Danger Canty @ http://6brand.com
Released under the same licence as Rails (MIT)