sqlite schema version check

    def check_schema(self):
        if self.debug:
            print('storage: check_schema')

        try:
            result = self.execute('get_metadata', name='version').fetchone()
            self.version = int(result[0])
            if self.debug:
                print('version read as', self.version)

        except (sqlite3.OperationalError, TypeError):
            self.version = 0
            if self.debug:
                print('version set to', self.version)

        for schema in (s for s in self._doc.iter('schema')
            if int(s.get('version')) > self.version
            ):
            self.version = self.upgrade_schema(schema)
            if self.debug:
                print('upgraded schema, ver', self.version)