java - Datastore Projection Queries on key/id in

Question

I have a model, say,

class MainModel {
   @Id String id;
   String p1;
   String p2;
   int p3;
   /... some 10-15 fields
}

In a query, all I want is values of p1 for some ids. So, basically,

SELECT __key__, p1 from MainModel where __key__ in [ 'i1', 'i2', ... ]

What's the best way to do this in Java?