Let's not put "rightness" in quotes :-)

I could see a `UsedIfRequested` or a `HasRelatedResource` as little mixins. Two troubles are (1) you still need to pass params through the constructor so the abstraction is leaky so doesn't buy you much and (2) testing; a mixin is really a function from `Class -> Class` so the domain under test is arbitrary enough that you can't really be sure of anything so you've got to document a lot of caveats.

Combinators are the better version of mixins, just as composition is the better version of inheritance, so something like this would be sweet:

```
class XFormInstanceResource(Resource):
   cases_field = ListDehydratedVia(CommCareCaseResource, UsedIfRequested(ListField('cases')))
```

aka `cases_field = (ListDehydratedVia(CommCareCaseResource) ∘ UsedIfRequested() ∘ ListField) ('cases')`

But obvs this isn't how Tastypie rolls by default so I didn't think of it. Let's save it for a refactor?