Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'authors') and self.authors is not None:
            _dict['authors'] = [x._to_dict() for x in self.authors]
        if hasattr(self,
                   'publication_date') and self.publication_date is not None:
            _dict['publication_date'] = self.publication_date
        if hasattr(self, 'title') and self.title is not None:
            _dict['title'] = self.title
        if hasattr(self, 'image') and self.image is not None:
            _dict['image'] = self.image
        if hasattr(self, 'feeds') and self.feeds is not None:
            _dict['feeds'] = [x._to_dict() for x in self.feeds]
        return _dict