Add Attribute In Activerecord

I'm trying to add an extra attribute in my model, but without success.

Added to the model attribute:

...
public $extraAttr;
...

and safe in rules

...
array('extraAttr', 'safe'),
...

Even so, this attribute does not appear in:

$model->attributes

I can only access it by:

$model->extraAttr

But I need to access it by aattributes the model. How do?

Thank you!