I have an lookup field in my opportunity object which is pointing to a custom object called Code__c.
Code__c has an external Id field called Code_Source__c.
When creating an opportunity once way of linking the opportunity to Code__c is by passing Id.
Opportunity op = new Opportunity(Name = 'Test Op', StageName = 'Open', ... , Oppoortunity_Code__c =code.Id );
Is it possible to use external ID (Code_Source__c) instead of the code.Id?