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,
                   'cross_reference') and self.cross_reference is not None:
            _dict['cross_reference'] = self.cross_reference
        if hasattr(self, 'relevance') and self.relevance is not None:
            _dict['relevance'] = self.relevance
        return _dict