class DatabaseChecker:
    implements(checkers.ICredentialsChecker)
    credentialInterfaces = credentials.IUsernamePassword, credentials.IUsernameHashedPassword

    def __init__(self, cp):
        self.cp = cp

    def requestAvatarId(self, creds):
        def _((matched, avatarId)):
            if matched:
                return avatarId
            else:
                return failure.Failure(error.UnauthorizedLogin())

        return getOperator(self.cp, creds.username
            ).addErrback(lambda _: failure.Failure(error.UnauthorizedLogin())
            ).addCallback(lambda operator: defer.maybeDeferred(creds.checkPassword, operator.password
                ).addCallback(lambda matched: (matched, operator))
            ).addCallback(_)

def getOperator(cp, username):
    def _(results):
        for row in results:
            return Operator(*row)
        raise ValueError('No such operator.')

    return cp.runQuery('SELECT username, password FROM operators WHERE username = %s', username
        ).addCallback(_)