Method _construct_instance

cassandra/cqlengine/models.py:449–489

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

(cls, values)

    @classmethod
    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
        if cls._db_map:
            values = dict((cls._db_map.get(k, k), v) for k, v in values.items())

        if cls._is_polymorphic:
            disc_key = values.get(cls._discriminator_column_name)

            if disc_key is None:
                raise PolymorphicModelException('discriminator value was not found in values')

            poly_base = cls if cls._is_polymorphic_base else cls._polymorphic_base

            klass = poly_base._get_model_by_discriminator_value(disc_key)
            if klass is None:
                poly_base._discover_polymorphic_submodels()
                klass = poly_base._get_model_by_discriminator_value(disc_key)
                if klass is None:
                    raise PolymorphicModelException(
                        'unrecognized discriminator column {0} for class {1}'.format(disc_key, poly_base.__name__)
                    )

            if not issubclass(klass, cls):
                raise PolymorphicModelException(
                    '{0} is not a subclass of {1}'.format(klass.__name__, cls.__name__)
                )

            values = dict((k, v) for k, v in values.items() if k in klass._columns.keys())

        else:
            klass = cls

        instance = klass(**values)
        instance._set_persisted(force=True)
        return instance

    def _set_persisted(self, force=False):
        # ensure we don't modify to any values not affected by the last save/update