Tosca Test Case Design relationships with Classes

I am doing test case design on the Sample Application sampleapp.tricentis.com. I want to do the test design in the most re-useable way possible, so i have split it into classes.

I want to create a relationship to yield all instances of the Fuel Type and Mileage, however when i do this, it informs me that This is a class reference, you must resolve this reference. This runs contrary to my need to have a modular test design. I am unable to create relationships on the class level, so what is the best way to resolve this?

@David I have faced similar constraints where I had to balance reusability and adding relationships. But TOSCA allows relationships at TestSheet level. it cannot use classes within releationships and that is the answer and you would know it better.

But what I did was to create a class and create instances within that class which is unique to the requirement of the class. Then use it within TS without relationships. And delete the combinations which are not required(forbidden relationship), as other required and constrained will be part of the generated combinations. Long way to achieve what we can achieve with relationships but its working for me till date.