Return a json dictionary representing this model.

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