Error checking for nested writes

[kaseyb002]

I believe this issue was partially addressed in #2263, but I ran into this issue again with a many to many field.

class Testy(models.Model):
    user_list = models.ManyToManyField('auth.User', related_name='just_test', blank=True)

class TestySerializer(serializers.ModelSerializer):
     user_list = serializers.SlugRelatedField(slug_field='id', many=True, queryset=User.objects.all())
     user_list_detail = UserSerializer(source='user_list', many=True, read_only=True)
     
     class Meta:
         model = Testy
         fields = (
             'id',
             'user_list',
             'user_list_detail',
)

AssertionError: The `.create()` method does not support writable nested fields by default.
Write an explicit `.create()` method for serializer `timeslots.serializers.TestySerializer`, or set `read_only=True` on nested serializer fields.

As shown, I've set the nested serializer field as read_only=True. When I delete the user_list_detail field, everything works fine. My goal is to write into the user_list field using ids, but be able to read the user list fields using UserSerializer (to get the depth).

This problem does not occur with ForeignKey fields. Additionally, reading works fine with the user_list_detail field included, but writes do not work.

[kevin-brown]

This sounds like a duplicate of #4634, which was recently fixed in #4669.