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All you have to do is set the keys when you allocate the entities: Entity entity = new Entity(key); Or if you had previously pulled the entities from the datastore, the keys should already be set.

Developers building applications on top of Java App Engine can use the familiar java.net interface for making off-network calls. For simple requests, this should be more than sufficient. The low-level API, however, provides one feature not available in java.net: asynchronous URLFetch.

The one killer feature of App Engine’s low-level API that isn’t present in java.net is asynchronous URLFetch. What is asynchronous fetch? Let’s make an analogy:

Have you considered doing both? Then you could quickly get a list of computers a student owns by key OR use a query which returns results in some sorted order. I don't think maintaining a list of keys on the student model is as intimidating as you think.