docs: 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