[storm] referenceset and lazy attributes

[Gustavo Niemeyer]

> if we have somekind of grouping we would also have a solution for the
> "fetch only primary-keys in referencesets" use-case. currently it
> selects all fields for getting the result, without knowing if the
> object is already cached.
>
> with only a lazy keyword this cannot be done.

This will be tackled by another improvement, which actually won't
require any changes in the ReferenceSet implementation nor in the
caching mechanism.  The Store will be improved to always select
only primary keys, and then select data for missing objects which
are not yet cached.  Sum that and the caching implementation we're
planning, we'll get a nice performance boost everywhere, and
ReferenceSets will be happy for free.

-- 
Gustavo Niemeyer
http://niemeyer.net