class LastUserField(models.ForeignKey):
    """                                                                      
    A field that keeps the last user that saved an instance                  
    of a model. None will be the value for AnonymousUser.                    
    """

def __init__(self, **kwargs):
    models.ForeignKey.__init__(self, User, null=True, **kwargs)

def contribute_to_class(self, cls, name):
    super(LastUserField, self).contribute_to_class(cls, name)
registry = registration.FieldRegistry(self.__class__)
registry.add_field(cls, self)
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