Association/Navigation without key properties

Hi Experts,

I'm trying to make an association/navigation to another Entity, but this for a property without being marked as a key.

Example the EPM test service from http://scn.sap.com/docs/DOC-31221

The model: http://scn.sap.com/servlet/JiveServlet/showImage/102-44073-2-246387/Relationen.png

I already builded the SalesOrder association/navigation to the SalesOrderItem.

But I cannot seem to create the association/navigation from SalesOrderItem to the Product, in SEGW I always have to define the referential constraint with the key of the Entity. In SalesOrderItem, the productId property is not a key, and I can't imagine that I have to set it as a key.

Thanks in advance

Regards,

Wouter