Realm should also support composite primary keys. Reusing the @PrimaryKey annotation should be relatively simple.

public class CompositeKeyObj extends RealmObject {

  @PrimaryKey
  private int compositePartOne;

  @PrimaryKey
  private int compositePartTwo;
}

Primary keys are currently implemented in the binding. This should be implemented at the core level: https://github.com/realm/realm-core/issues/831