Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'document_id') and self.document_id is not None:
            _dict['document_id'] = self.document_id
        if hasattr(self, 'field') and self.field is not None:
            _dict['field'] = self.field
        if hasattr(self, 'start_offset') and self.start_offset is not None:
            _dict['start_offset'] = self.start_offset
        if hasattr(self, 'end_offset') and self.end_offset is not None:
            _dict['end_offset'] = self.end_offset
        if hasattr(self, 'entities') and self.entities is not None:
            _dict['entities'] = [x._to_dict() for x in self.entities]
        return _dict