Add ModelWithFieldAttributes *before* PolyModel, so that everything does not become a "ModelWithFieldAttributes" in the Datastore. The way PolyModel works, each Model class hierarchy is stored in the Datastore as the class that directly inherited from PolyModel. So, we do not want to insert ModelWithFieldAttributes *after* PolyModel by inheriting *from* PolyModel, or everything would just be a ModelWithFieldAttributes in the Datastore, instead of Person, Work, Group, etc.

"""Error types for dispatcher mechanism
"""

class DispatcherError(Exception):
    """Base class for all Dispatcher errors"""
class DispatcherKeyError(KeyError, DispatcherError):
    """Error raised when unknown (sender,signal) set specified"""
class DispatcherTypeError(TypeError, DispatcherError):
    """Error raised when inappropriate signal-type specified (None)"""