Wrong handling of initial value on disabled ModelMultipleChoiceField

[karyon]

So i provided a queryset as initial value for a non-required and disabled ModelMultipleChoiceField. it seemed to work as i saw the correct values in the rendered output.

but, when saving the form i got an error Enter a list of values.

1. this happens because disabled fields send their initial value to cleaning, see [​here. in case of model fields, it looks like this must be a list of PKs, which is quite unhandy to build (something like list(queryset.values_list('pk', flat=True)))
2. the error did not show up when the queryset i provided yielded no results, which is inconsistent. the reason for that is that ​this boolean expression evaluates to true as soon as there are results in the queryset.

3. something else appeared fishy to me: in one line the clean method returns a queryset (self.queryset.none()), but in the next line it raises an error if the value is neither a list nor a tuple.

if you folks tell me what the intended behaviour is / which types should be used, i can try to create a fix if you want.

4. and the last (slightly unrelated) issue: since the field was disabled, i was wondering why cleaning it doesn't take a shortcut and does nothing at all... bumped into this a few times.