def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'cell_id') and self.cell_id is not None:
            _dict['cell_id'] = self.cell_id
        if hasattr(self, 'location') and self.location is not None:
            _dict['location'] = self.location._to_dict()
        if hasattr(self, 'text') and self.text is not None:
            _dict['text'] = self.text
        if hasattr(self,
                   'row_index_begin') and self.row_index_begin is not None:
            _dict['row_index_begin'] = self.row_index_begin
        if hasattr(self, 'row_index_end') and self.row_index_end is not None:
            _dict['row_index_end'] = self.row_index_end
        if hasattr(
                self,
                'column_index_begin') and self.column_index_begin is not None:
            _dict['column_index_begin'] = self.column_index_begin
        if hasattr(self,
                   'column_index_end') and self.column_index_end is not None:
            _dict['column_index_end'] = self.column_index_end
        if hasattr(self, 'row_header_ids') and self.row_header_ids is not None:
            _dict['row_header_ids'] = [
                x._to_dict() for x in self.row_header_ids
            ]
        if hasattr(self,
                   'row_header_texts') and self.row_header_texts is not None:
            _dict['row_header_texts'] = [
                x._to_dict() for x in self.row_header_texts
            ]
        if hasattr(self, 'row_header_texts_normalized'
                  ) and self.row_header_texts_normalized is not None:
            _dict['row_header_texts_normalized'] = [
                x._to_dict() for x in self.row_header_texts_normalized
            ]
        if hasattr(self,
                   'column_header_ids') and self.column_header_ids is not None:
            _dict['column_header_ids'] = [
                x._to_dict() for x in self.column_header_ids
            ]
        if hasattr(
                self,
                'column_header_texts') and self.column_header_texts is not None:
            _dict['column_header_texts'] = [
                x._to_dict() for x in self.column_header_texts
            ]
        if hasattr(self, 'column_header_texts_normalized'
                  ) and self.column_header_texts_normalized is not None:
            _dict['column_header_texts_normalized'] = [
                x._to_dict() for x in self.column_header_texts_normalized
            ]
        if hasattr(self, 'attributes') and self.attributes is not None:
            _dict['attributes'] = [x._to_dict() for x in self.attributes]
        return _dict
Return a json dictionary representing this model.