fields.py 文件源码

def __get__(self, instance, owner):
        """Descriptor to allow lazy dereferencing."""
        if instance is None:
            # Document class being used rather than a document object
            return self

        # Get value from document instance if available
        value = instance._data.get(self.name)
        self._auto_dereference = instance._fields[self.name]._auto_dereference
        # Dereference DBRefs
        if self._auto_dereference and isinstance(value, DBRef):
            if hasattr(value, 'cls'):
                # Dereference using the class type specified in the reference
                cls = get_document(value.cls)
            else:
                cls = self.document_type
            dereferenced = cls._get_db().dereference(value)
            if dereferenced is None:
                raise DoesNotExist('Trying to dereference unknown document %s' % value)
            else:
                instance._data[self.name] = cls._from_son(dereferenced)

        return super(ReferenceField, self).__get__(instance, owner)