viewsets.py

class CreateWithUserMixin(object):
    def create(self, request, *args, **kwargs):
        if request.DATA.get('user') is None:
            request.DATA['user'] = request.user.pk
        if request.DATA.get('last_changed_by') is None:
            request.DATA['last_changed_by'] = request.user.pk
        return super(CreateWithUserMixin, self).create(request, *args, **kwargs)


class UpdateWithUserMixin(object):
    def update(self, request, *args, **kwargs):
        if request.DATA.get('last_changed_by') is None:
            request.DATA['last_changed_by'] = request.user.pk
        return super(UpdateWithUserMixin, self).update(request, *args, **kwargs)