Prevent attribute from being changed with "attr_readonly"

Is there a way to prevent an attribute from being changed from outside
the class instance? So that after a record is created, the attribute can
be read but cannot be changed from outside the class instance.

At first glance, "attr_readonly" seems to be the solution. However,
will "attr_readonly" also prevent the attribute from being changed even
from inside the class?

In other words, will the following method successfully change the values
of attribute_a and attribute_b in model A?

1.Class A < ActiveRecord::Base
2. attr_readonly :attribute_a, :attribute_b
3. def change_attributes(new_a, new_b)
4. self.attribute_a = new_a
5. self.attribute_b = new_b
6. save!
7. end
8. end

I know I can redefine the write accessor if I want to customize the way
a single attribute is changed. However here I would like to change two
attributes in one transaction and prevent each attribute from being
changed individually.

Thanks

I tried to make private the write accessor for an attribute (e.g., foo)
that I want to protect by defining:

Code : - fold - unfold

   1. def foo=val
   2. super
   3. end

However, when I make the write accessor private, I cannot even call it
from inside the class because a statement such as self.foo = val returns
NoMethodError: Attempt to call private method
Please help. Thanks.