'Get all instance of a class hierarchy.
Args:
kwds: Keyword parameters passed on to Model.all.
Returns:
Query with filter set to match this class\' discriminator.'

@classmethod
def all(cls, **kwds):

    query = super(PolyModel, cls).all(**kwds)
    if (cls != cls.__root_class__):
        query.filter((_CLASS_KEY_PROPERTY + '    ='), cls.class_name())
    return query
