Relation to a module model

I am using a User module which provides a model for managing users (stored in: /protected/modules/user/models/User.php). In my own Article model I have a field author_id which refers to the User model from a User module (tricky, huh?).

How do I add a relation for this in my model? I have tried

'author' => array(self::BELONGS_TO, 'User', 'author_id')

but it raises PHP Error

include(User.php) [<a href='function.include'>function.include</a>]: failed to open stream: No such file or directory

whether you import the user model class or the whole module



thanks for your help mbi!