Return a json dictionary representing this model.

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
        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
        return _dict