Django REST Framework - create child of nested relationship

Question

I'm building an API with the Django Rest Framework for a site that allows users to create posts, and comment on those posts. I'm able to create posts from the API fine, but when I try to create a comment I get the following error:

NOT NULL constraint failed: app_comment.post_id

my models:

class Post(models.Model):
    owner = models.ForeignKey('auth.User', related_name = 'posts')
    post_title = models.CharField(max_length=200)
    post_description = models.CharField(max_length=1000)

class Comment(models.Model):
    user = models.ForeignKey(User, related_name = 'comments')
    post = models.ForeignKey(Post, related_name = 'comments')
    text = models.CharField(max_length=1000)

and my comment serializer:

class CommentSerializer(serializers.ModelSerializer):
    user = serializers.ReadOnlyField(source='user.username')
    post = serializers.ReadOnlyField(source='post.id')
    class Meta:
        model = Comment
        fields = ('id', 'text', 'user', 'post')

and my comment view:

class PostCommentList(generics.ListCreateAPIView):
    permission_classes = (permissions.IsAuthenticated,
                            IsOwnerOrReadOnly,)
    serializer_class = CommentSerializer
    def get_queryset(self):
        post = self.kwargs['post_pk']
        post = Post.objects.get(pk = post)
        return post.comments.all()
    def perform_create(self, serializer):
        post = self.kwargs['post_pk']
        print("creating a comment from " + str(self.request.user) + " on post " + str(post) +" : "+ str(Post.objects.get(pk = post)))
        serializer.save(user = self.request.user)
        serializer.save(post = self.kwargs['post_pk'])

When attempting to create a comment on a post, I see the correct information being printed out (i.e. 'creating a comment from user on post 6 : Post object').

Why are my comments not being created with the correct post_id?

Why are you overloading get_queryset and perform_create? I guess it should work normally if you don't do it. Could you check it, please? — Danilo Freitas
– Danilo Freitas, Commented Jan 14, 2016 at 18:31
I'm overloading get_queryset so that the only the comments attached to a specific post will be returned, and I'm overloading perform_create so that I can get the post number form the url and the user from the request (similar to the example here: django-rest-framework.org/api-guide/generic-views). I'll try removing them, but I believe it will break other functionality. — KSmith
– KSmith, Commented Jan 14, 2016 at 18:54

Tristan · Accepted Answer · 2016-01-14 18:56:55Z

2

In your perform_create function you are getting the primary key which is an integer, not a Post object. Also, you don't want to call save twice - it will result in saving two models (assuming it doesn't throw validation errors - which in this case it is).

A better way to do this might be:

post_pk = self.kwargs['post_pk']
post = Post.objects.get(pk = post_pk)
serializer.save(user=self.request.user, post=post)

Also you'll probably want to add the text content to the model you are saving as well. So perhaps, so you may need to add/change it to read:

text = self.kwargs['text']
serializer.save(user=self.request.user, post=post, text=text)

While I'm thinking of it - you may want to save a call to 'get' and avoid a trip to the database and specify the foreign key directly. You can do that using this syntax:

post_pk = self.kwargs['post_pk']
# Note - we won't call '.get' here
serializer.save(user=self.request.user, post_id=post_pk, text=text)

edited Jan 14, 2016 at 18:56

answered Jan 14, 2016 at 18:51

Tristan

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2 Comments

KSmith Over a year ago

Didn't realize I could put multiple parameters in the save function, thanks! I don't need to add the text to save though, I believe because it is handled by my serializer class.

banky Over a year ago

I have a similar issue to this.Can you help look into this please?stackoverflow.com/questions/62655168/…

Mad Wombat · Accepted Answer · 2016-01-14 19:13:17Z

0

Well, the most obvious error is that you are calling save() twice, first with a user and the with a post. And in the first call it tries to create a comment without a post and causes the database to give you the failed constraint error. Replace

serializer.save(user = self.request.user)
serializer.save(post = self.kwargs['post_pk'])

with

serializer.save(user=self.request.user,
                post=int(self.kwargs['post_pk']))

and you should be fine.

That said, you should probably remove the ReadOnlyFields and just add read_only_fields to the serializer meta

class CommentSerializer(serializers.ModelSerializer):
    class Meta:
        model = Comment
        fields = ('id', 'text', 'user', 'post')
        read_only_fields = ('post', 'user')

answered Jan 14, 2016 at 19:13

Mad Wombat

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1 Comment

banky Over a year ago

This looks similar to the issue I'm having presently. Can you help look into it please stackoverflow.com/questions/62655168/…

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