Return a json dictionary representing this model.

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