I think the reason the imports are weird is because of circular references--you're essentially wanting to check that nothing references this object, and so would need to import ___all models___ that reference Zone--which immediately leads to circular references.

I think we want to use something more generic, like this:
http://stackoverflow.com/questions/7932053/how-can-i-get-all-relatedmanagers-from-a-particular-django-model-instance