Unexpected behavior when handling an exception from serializer.is_valid()

[cbertinato via Django REST framework]

As the subject indicates, I am observing some unexpected behavior when
using a serializer within another.

I'm still working on knocking up a working example, but the setup is as
follows. I have a serializer that has a to_representation method and in
that serializer's update method I am serializing an instance of another
model. Something like:

class ParentSerializer:
class Meta:
model = SomeModel

def to_representation(self, obj):
...

def update(self, instance, validated_data):
serializer = SomeOtherModelSerializer(SomeOtherModel, data=data,
context=self.context, partial=True)
serializer.is_valid(raise_exception=True)
serializer.save()

If that serializer does not validate and raises an exception, then the
exception handler eventually calls on to_representation to render a
response. What is curious is that I see a call to the ParentSerializer
to_representation with an instance of SomeOtherModel. I only noticed
because the assumption in to_representation is that obj is only ever an
instance of SomeModel, which is throwing other errors.

Just trying to get a bead on whether this is expected behavior, or whether
this is indeed a issue in DRF. In the meantime I'll try to work up a small
example to replicate this issue.

Cheers,
Chris