That logic has to go somewhere right? I'm not sure what the issue is.

There's the "heuristic" of expecting a property to not be expensive to access (which django already kind of throws out the window depending on how you fetch the model), but otherwise I don't see how services fixes this. Is copy and pasting that code over to a services.py file better?

Services are very nice for dealing with python's import issue. Accessing other models from a method / property on a model is ugly. But it's very hard to structure and name services in a logical way, especially when the lines start to cross. You end up with a "shared_services.py".