code: def build_db
        build_config()
        unless @config['key_fields'].keys.select { |k| @config['key_fields'][k]['required'] }.count > 0
          raise SpreadsheetAgentError, "Your configuration must have at least one required key_fields key"
        end
        @session = GoogleDrive.login(@config['guser'], @config['gpass'])
        @db = @session.spreadsheet_by_title(@config['spreadsheet_name'])
      end
docs: This is for internal use by SpreadsheetAgent classes that extend SpreadsheetAgent::Db