MultiModelField fails when validating large Querysets

[peterfarrell]

When using large Querysets as the input to MultipleModelField, the service fails (usually to out of memory / timeouts). This occurs because the Queryset is evaluated in clean as the MultipleModelField tries to evaluate that all items in the iteration are of type app.Thing.

This breaks even if you are handling large querysets with chunking in the process method.

class DoSomethingToThings(Service):
    things = MultipleModelField("app.Thing", required=True)

    def process(self):
        for thing in Model.objects.all().iterator(chunk_size=2000):
            pass  # Do something

Not sure how to handle this in MultipleModelField as iterable datatypes like list and tuples in addition to Django Querysets. Also, it has an unsaved check for model objects.

Propose a QuerysetField that accept Django Querysets of a certain type only. I will make a PR.