How can I get a trigger to fire for the first time and not each time

[Sam]

Hello - I'm trying to track first response time for a project in Jira.  I can get it to work by either using a trigger on when a transition occurs or its assignee changes.  However if I repeat those triggers, it updates the time variables again.  

Is there a way to trigger an assignee change only when it goes from blank to assigned?

Thanks

[Alexander Pappert · Accepted solution]

https://www.atlassian.com/software/jira/automation-template-library/rules#/labels/all/customLabelId/1453

condition: assignee = uassigned