Django REST Framework different depth for POST/PUT?

I am using Django REST Framework to create an API for my web app. I have a class 'Comment', that has depth=2 set in the Meta class. This works great when GETing the Comments. When I try to send a POST or PUT request though (i.e. create a new Comment) I am told I need to include objects instead of ForeignKey IDs.

Here's my Serializer class:

class CommentSerializer(serializers.ModelSerializer):
    class Meta:
        model = Comment
        depth = 2 

The model:

class Comment(models.Model):
    user = models.ForeignKey(User, null=True, blank=True,
        related_name='comments')
    budget = models.ForeignKey(Budget, related_name='comments')
    published = models.BooleanField(default=False)
    body = models.TextField()
    created = models.DateTimeField(auto_now_add=True)

The view code:

class Comments(generics.ListCreateAPIView):

    model = Comment
    serializer_class = CommentSerializer

    def pre_save(self, obj):
        obj.user = self.request.user

And the error that is displayed in the output (JSON) is:

{"user": ["This field is required."], "budget": [{"non_field_errors": ["Invalid data"]}]}

When this raw data is sent:

{"budget": 2, "published": true, "body": "Another comment"}