That's correct (although from your example I would use $get(%request.Data("Q31",1)) over the "Get" method). I had another look and an assumption I had made in my previous post was partially incorrect. At least in questionnaires you can safely check the IsNew property of %UDF - this is correctly set when a questionnaire is created using either an API or through the UI. This value is derived from a variable that is set at the very top of the save so it allows us to identify a new added record. Here is a fairly simple (if a little crude) example:

ClassMethod Test()
{
	tstartdo..GetAPI(.objAPI)
	set objAPI.QUESPAPatMasDR = 5878set objAPI.QUESPAAdmDR = 11480set objAPI.Q01 = "Y"set rc = objAPI.Save()
	zwrite {"Q01":(objAPI.Q01)}
	set id = objAPI.ID
	kill objAPI
	set obj = ##class(questionnaire.QAUXXARP).%OpenId(id)
	
	zwrite {"Q01":(obj.Q01)}
	kill obj
	do..GetAPI(.objAPI)	
	set rc = objAPI.Open("",id)
	set objAPI.Q01 = "Y"set rc = objAPI.Save()
	zwrite {"Q01":(objAPI.Q01)}
	kill objAPI
	set obj = ##class(questionnaire.QAUXXARP).%OpenId(id)
	zwrite {"Q01":(obj.Q01)}
	trollback
}

ClassMethod OnAfterSave() As%Boolean
{
	#dim%UDFas websys.UDFExt.TrakEvent
	set sc = $$$OKwrite !,"%UDF.IsNew = "_%UDF.IsNew,!
	if%UDF.IsNew {
		set obj = ##class(questionnaire.QAUXXARP).%OpenId(%UDF.Id,,.sc)
		if$isobject(obj) {
			set obj.Q01 = "N"set sc = obj.%Save()
		}
	}
	if$$$ISERR(sc) {
		do%UDF.Message.Status(sc)
	}
	return1
}

do##class(Custom.AUXX.UDF.questionnaire.QAUXXARP).Test()

%UDF.IsNew = 1
{"Q01":"Y"}  ; <DYNAMIC OBJECT>
{"Q01":"N"}  ; <DYNAMIC OBJECT>%UDF.IsNew = 0
{"Q01":"Y"}  ; <DYNAMIC OBJECT>
{"Q01":"Y"}  ; <DYNAMIC OBJECT>

As you can see, the value of "Q01" is only set to "N" on the first save and is not updated afterwards.

Ah. Thanks for clarifying. That makes a bit more sense now but also raises another question. Are you creating the questionnaire through an API call, or is the questionnaire being added/updated as a result of an API call to a different component? If it's the former, you should just be able to populate the property when it's created. 

I can't think of any reason why OnBeforeSave wouldn't fire in any case. I can have a look on Monday when I'm in front of my work computer. I've done similar to what you're trying to do and I would usually set a flag using a PPG in OnBeforeSave if %request.Data("ID",1)="" and then check that in the OnAfterSave and update the item then. Alternatively if there's no ID (like above), you could set %request.Data(<component item name>,1) = 1 in OnBeforeSave and the %request value would be used when it populates the object values to save.