Create Calculated Property in Persistent Class based on Relationship

I don't think you can reference relationships in SqlComputeCode, so I'm not sure how you'd do this. But out of curiosity, why have a calculated field in the question class rather than just referring to it as Document.FileName (or Document->FileName in SQL)?

Here's a possible alternative to using SqlComputeCode.

You can override the getter method for that property so that it always returns the related document name. In your Question class, define your docFileName property as:

Property docFileName As %String [ReadOnly];

You'll want it to be read only because you aren't going to want to be able to set this property in the question object. You're going to want to retrieve it from the document object. If it could also be set here, you'd have things weirdly out of sync. Then, in that same class, include the following method:

Method docFileNameGet() As %String{
    if ..Document '= ""{
        return ..Document.FileName
    }
    else{
        return ""
    }
}

Once you do that, whenever you refer to the question's docFileName, it'll give you the document's file name, or an empty string if there isn't one.

Another option 

/// This sample persistent class represents an employee.<br>
Class Sample.Employee Extends Person
{

...

/// The company this employee works for.
Relationship Company As Company [ Cardinality = one, Inverse = Employees ];

Property Cname As %String [ Calculated, SqlComputeCode = {n id,r s id={%%ID} &sql(select Company->Name into :r from Sample.Employee where %ID=:id) s {*}=r}, SqlComputed ];

...
}

You do it by chaining GetStored methods:

Class Document Extends %Persistent {

Property Filename As %VarString;

}

Class Question As %Persistent {

// Or relationship
Property Document As Document;

Property Filename As %String [ Calculated, SqlComputeCode = {set {*}=##class(Document).FilenameGetStored({Document})}, SqlComputed ];

}