%OpenId Query

use instead.

set ID=##Class(User.School).%OpenId(RollNo_"||"_Marks,,.sc)

do $system.OBJ.DisplayError(sc)

now you should see the reason why  ID=""

docs: https://cedocs.intersystems.com/latest/csp/documatic/%25CSP.Documatic.cls?PAGE=CLASS&LIBRARY=SAMPLES&CLASSNAME=%25Library.Persistent#%OpenId

When you have || in Id, it usually means, that you have additional Index by two or more fields, and marked as IdKey. Something like this.

Property RollNo As %String;

Property Marks As %String;

Index RollNoMarks On (RollNo, Marks) [ IdKey, Unique ];

Here we have some two properties, to store both values separately, but we used both of their values as IdKey.

Now it is possible to check of existing an object by ##class(User.School).%ExistsId(RollNo_"||"_Marks), or open id by  ##class(User.School).%OpenId(RollNo_"||"_Marks)

But in additional to this system methods, we now have some new ones which construct with index name and "Exists", "Open" and so on. But call will be a bit different. You should pass all values separately in this way.

##class(User.School).RollNoMarksExists(RollNo, Marks)

##class(User.School).RollNoMarksOpen(RollNo, Marks, , .sc)

You can try any of these ways, and you should remember about Status variable, which may help you to understand what's happening.

do $system.OBJ.DisplayError(sc)

will decode status