How do I add a foreign key field in a post method for Django Rest Framework

Question

I am really sorry if I am being a noob, but how do I add a foreign key field with my post method. I need to mention the blog post, the user who made the comment with the post. But, the post and the user/owner are both foreign key fields in my models. With Postman it gives a Django Integrity error.

models.py

class Comment(models.Model) :
    text = models.TextField(
        validators=[MinLengthValidator(3, "Comment must be greater than 3 characters")]
    )

    post = models.ForeignKey(Post, on_delete=models.CASCADE)
    owner = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)

    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    # Shows up in the admin list
    def __str__(self):
        return (self.post.title)

serializers.py

class CommentSerializer(serializers.ModelSerializer):
    owner = serializers.StringRelatedField()
    post = serializers.StringRelatedField()

    class Meta:
        model = Comment
        fields = ['id', 'text', 'created_at', 'post', 'owner']

views.py

class CommentViewSet(viewsets.ModelViewSet):
    serializer_class = CommentSerializer
    permission_classes = [permissions.IsAuthenticatedOrReadOnly]
    queryset = Comment.objects.all()
    filter_backends = [DjangoFilterBackend]
    filter_fields = ['post']

Full error trace:

 File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/backends/sqlite3/base.py", line 413, in execute
    return Database.Cursor.execute(self, query, params)
sqlite3.IntegrityError: NOT NULL constraint failed: api_comment.owner_id

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/core/handlers/base.py", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/rest_framework/viewsets.py", line 125, in view
    return self.dispatch(request, *args, **kwargs)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/rest_framework/views.py", line 509, in dispatch
    response = self.handle_exception(exc)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/rest_framework/views.py", line 469, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/rest_framework/views.py", line 480, in raise_uncaught_exception
    raise exc
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/rest_framework/views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/rest_framework/mixins.py", line 19, in create
    self.perform_create(serializer)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/rest_framework/mixins.py", line 24, in perform_create
    serializer.save()
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/rest_framework/serializers.py", line 205, in save
    self.instance = self.create(validated_data)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/rest_framework/serializers.py", line 939, in create
    instance = ModelClass._default_manager.create(**validated_data)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/models/query.py", line 447, in create
    obj.save(force_insert=True, using=self.db)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/models/base.py", line 753, in save
    self.save_base(using=using, force_insert=force_insert,
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/models/base.py", line 790, in save_base
    updated = self._save_table(
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/models/base.py", line 895, in _save_table
    results = self._do_insert(cls._base_manager, using, fields, returning_fields, raw)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/models/base.py", line 933, in _do_insert
    return manager._insert(
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/models/query.py", line 1254, in _insert
    return query.get_compiler(using=using).execute_sql(returning_fields)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/models/sql/compiler.py", line 1397, in execute_sql
    cursor.execute(sql, params)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/backends/utils.py", line 98, in execute
    return super().execute(sql, params)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/backends/utils.py", line 66, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/backends/utils.py", line 75, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/Users/syedzayyanmasud/Desktop/blog/venv/lib/python3.9/site-packages/django/db/backends/sqlite3/base.py", line 413, in execute
    return Database.Cursor.execute(self, query, params)
django.db.utils.IntegrityError: NOT NULL constraint failed: api_comment.owner_id

Answer 1

You need to change your serializer as below

from rest_framework.fields import CurrentUserDefault

class CommentSerializer(serializers.ModelSerializer):
    owner = serializers.StringRelatedField()
    post = serializers.StringRelatedField()

    def create(self, validated_data):
        obj = Comment(**validated_data)
        obj.owner = CurrentUserDefault()
        obj.save()
        return obj

    class Meta:
        model = Comment
        fields = ['id', 'text', 'created_at', 'post', 'owner']

You need to import the required imports.

ref: https://www.django-rest-framework.org/api-guide/validators/#currentuserdefault