Update method of serializer only creates and doesn't update (Django rest framework) - django-rest-framework

Question

I'm having a hard time with nested serializers, especially updating them. So far I can update the lesson description and all the main fields before the nested serializer (like course description, title etc). Ideally I'd like to search using lesson_id, and not lesson.title like it is now, and update description and title at the same time. Is there a workaround for that?
My models.py
class Course (models.Model):
title = models.CharField (max_length=150)
description = models.CharField(max_length=250, default="No Description")
student = models.ManyToManyField(Student, related_name='courses', blank=True)
teacher = models.ManyToManyField(Teacher, related_name='teacher', blank=True)
def __str__(self):
return self.title
class Lesson (models.Model):
title = models.CharField(max_length=100)
description = models.TextField(default="No Description")
course = models.ForeignKey(Course, on_delete=models.CASCADE, related_name='lessons')
def __str__(self):
return self.title
Serializers.py where the problem is
class CourseSerializer (serializers.ModelSerializer):
lessons = LessonSerializer(many=True, required=False)
#teacher = TeacherSerializer(many=True)
class Meta:
model = Course
fields = ('id', 'title', 'description', 'lessons') #to separate serializer with students for teachers later
def update(self, instance, validated_data):
lessons = validated_data.pop('lessons', [])
instance = super().update(instance, validated_data)
for lesson in lessons:
lesson, updated = Lesson.objects.update_or_create( defaults={'description': lesson["description"]}, title= lesson["title"])
#pk = instance.lessons_id doesn't work, I cannot get this id
instance.save()
return instance
Views
class CourseDetailDeleteView (generics.RetrieveUpdateDestroyAPIView):
permission_classes = (IsAdminUserOrAuthenticatedOrReadOnly,)
queryset = Course.objects.all()
serializer_class = CourseSerializer
def update(self, request, *args, **kwargs):
serializer = CourseSerializer(instance=self.get_object(), data=request.data, partial=True)
if serializer.is_valid():
serializer.save()
return Response(serializer.data)

Answer 1

If your problem is that validated_data["lessons"][0]["id"] doesn't exists:
By default the id field of a ModelSerializer is read_only, so id is not included in validated_data in .create() or .update(). Thus you'll have to override that:
class LessonSerializer(serializers.ModelSerializer):
id = serializers.IntegerField()
....
## Optional: in case you don't want `id` getting explicitly set
def create(self, validated_data):
validated_data.pop("id", None)
return super().create(self, validated_data)
## Optional: in case you don't want `id` of the instance getting updated
def update(self, instance, validated_data):
validated_data.pop("id", None)
return super().update(self, instance, validated_data)
Another note:
There's no way to know if serializer.save() is even called in CourseDetailDeleteView.update. I would suggest to set serializer.is_valid(raise_exception=True) instead so it can return error messages and show you why it didn't save.