ADO.NET Data Services quickly exposes a database as a RESTful web service, including query support. This is useful for AJAX applications, such as those built with ExtJS. Unfortunately, however,while ExtJS 3.0 has built-in REST support, its conventions don’t map well to Data Services. The most obvious case is composite primary keys (e.g., "/Store.svc/Cart(UserID=1,ProductID)"). This post discusses a simple way of accomodating these.