Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'document_label') and self.document_label is not None:
            _dict['document_label'] = self.document_label
        if hasattr(self, 'location') and self.location is not None:
            _dict['location'] = self.location._to_dict()
        if hasattr(self, 'text') and self.text is not None:
            _dict['text'] = self.text
        if hasattr(self, 'types') and self.types is not None:
            _dict['types'] = [x._to_dict() for x in self.types]
        if hasattr(self, 'categories') and self.categories is not None:
            _dict['categories'] = [x._to_dict() for x in self.categories]
        if hasattr(self, 'attributes') and self.attributes is not None:
            _dict['attributes'] = [x._to_dict() for x in self.attributes]
        return _dict