Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'language') and self.language is not None:
            _dict['language'] = self.language
        if hasattr(self, 'analyzed_text') and self.analyzed_text is not None:
            _dict['analyzed_text'] = self.analyzed_text
        if hasattr(self, 'retrieved_url') and self.retrieved_url is not None:
            _dict['retrieved_url'] = self.retrieved_url
        if hasattr(self, 'usage') and self.usage is not None:
            _dict['usage'] = self.usage._to_dict()
        if hasattr(self, 'concepts') and self.concepts is not None:
            _dict['concepts'] = [x._to_dict() for x in self.concepts]
        if hasattr(self, 'entities') and self.entities is not None:
            _dict['entities'] = [x._to_dict() for x in self.entities]
        if hasattr(self, 'keywords') and self.keywords is not None:
            _dict['keywords'] = [x._to_dict() for x in self.keywords]
        if hasattr(self, 'categories') and self.categories is not None:
            _dict['categories'] = [x._to_dict() for x in self.categories]
        if hasattr(self, 'emotion') and self.emotion is not None:
            _dict['emotion'] = self.emotion._to_dict()
        if hasattr(self, 'metadata') and self.metadata is not None:
            _dict['metadata'] = self.metadata._to_dict()
        if hasattr(self, 'relations') and self.relations is not None:
            _dict['relations'] = [x._to_dict() for x in self.relations]
        if hasattr(self, 'semantic_roles') and self.semantic_roles is not None:
            _dict['semantic_roles'] = [
                x._to_dict() for x in self.semantic_roles
            ]
        if hasattr(self, 'sentiment') and self.sentiment is not None:
            _dict['sentiment'] = self.sentiment._to_dict()
        if hasattr(self, 'syntax') and self.syntax is not None:
            _dict['syntax'] = self.syntax._to_dict()
        return _dict