method used to construct instances from query results
        this is where polymorphic deserialization occurs

def _construct_instance(cls, values):
        """
        method used to construct instances from query results
        this is where polymorphic deserialization occurs
        """
        # we're going to take the values, which is from the DB as a dict
        # and translate that into our local fields
        # the db_map is a db_field -> model field map
        items = values.items()
        field_dict = dict([(cls._db_map.get(k, k),v) for k,v in items])

        if cls._is_polymorphic:
            poly_key = field_dict.get(cls._polymorphic_column_name)

            if poly_key is None:
                raise PolyMorphicModelException('polymorphic key was not found in values')

            poly_base = cls if cls._is_polymorphic_base else cls._polymorphic_base

            klass = poly_base._get_model_by_polymorphic_key(poly_key)
            if klass is None:
                poly_base._discover_polymorphic_submodels()
                klass = poly_base._get_model_by_polymorphic_key(poly_key)
                if klass is None:
                    raise PolyMorphicModelException(
                        'unrecognized polymorphic key {} for class {}'.format(poly_key, poly_base.__name__)
                    )

            if not issubclass(klass, cls):
                raise PolyMorphicModelException(
                    '{} is not a subclass of {}'.format(klass.__name__, cls.__name__)
                )

            field_dict = {k: v for k, v in field_dict.items() if k in klass._columns.keys()}

        else:
            klass = cls

        instance = klass(**field_dict)
        instance._is_persisted = True
        return instance