Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'text') and self.text is not None:
            _dict['text'] = self.text
        if hasattr(self, 'relevance') and self.relevance is not None:
            _dict['relevance'] = self.relevance
        if hasattr(self,
                   'dbpedia_resource') and self.dbpedia_resource is not None:
            _dict['dbpedia_resource'] = self.dbpedia_resource
        return _dict