I have a custom object which draws an External ID field from its Parent. I created a Formula Field to pull that data in to the custom field. However, I want to use that field on the custom object as an External ID for the the custom object... but can't because of the limitation that formula field caanot be an external id. Is there another way I can accomplish this?
String externalId = 'SOMETHING'; ChildObject__c childObject = new ChildObject(); childObject.Account__r = new Account(ExternalId__c = externalId); insert childObject;
You don't need the external Id on the child record.
EDIT: Relevant documentation