def get_next_id(self):
        try:
            query = {'_id': 'last_id'}
            update = {'$inc': {'id': 1}}
            fn = self.db.internal.find_and_modify
            row = fn(query, update, upsert=True, new=True)
        except Exception as e:
            raise DatabaseException(e)
        return row['id']
def log_cell(self, row_category: str, col_category: str, value: int):
        """Logs a cell in the confusion matrix.

        Args:
          row_category: String representing the name of the row category.
          col_category: String representing the name of the column category.
          value: Int value of the cell.

        Raises:
          ValueError: If row_category or col_category is not in the list of
           categories set in set_categories.
        """
        if row_category not in self._categories:
            raise ValueError('Invalid category: {} passed. Expected one of: {}'.\
              format(row_category, self._categories))

        if col_category not in self._categories:
            raise ValueError('Invalid category: {} passed. Expected one of: {}'.\
              format(row_category, self._categories))

        self._matrix[self._categories.index(row_category)][
            self._categories.index(col_category)] = value