[syslog-ng] Check if field in message and use default if not.

I generate a Kafka topic to send the message to by doing: topic("${topic}.${environment}") What I'd like to do if the field wasn't in the json message to provide a default value. Im looking for like a tertiary operator to use In the topic or perhaps a rewrite rule. I see the conditional expressions but from the docs it doesn't look like I can use it in the topic or a rewrite rule NOTE: My messages are json stings that get parsed and then imploded back to json as it's sent. Thanks Ronald Fenner Network Architect Game Circus LLC. rfenner@gamecircus.com