Handling the various derivations of a product can be a daunting (and costly) task. To tackle this problem, we propose a method based on the use of a creational design pattern to uncouple the variations from the selection process. This makes it possible to automatically derive a given product from the set of all possible ones, and to specialize its model accordingly. The contribution of this paper is to provide a set of patterns for modeling variability issues of a Product Line Architecture to define architectural constraints for Product Line expressed in UML as meta-level OCL constraints and to propose an approach to automate the derivation process.