how can solve this: null value in column "course_id" of relation "exercise" violates not-null constraint

[xiayulu]

My files:

models.py

class Course(models.Model):
    name = models.CharField(max_length=63, unique=True)
    about = models.CharField(max_length=255, blank=True, null=True)

class Exercise(models.Model):

    title = models.CharField(max_length=63)
    course = models.ForeignKey('course.Course', on_delete=models.CASCADE, related_name='exercises')

serializers.py

class CourseSerializer(serializers.ModelSerializer):
    exercises = serializers.HyperlinkedRelatedField(
        many=True,
        read_only=True,
        view_name='exercise-detail')

    class Meta:
        model = Course
        fields = ['id', 'url', 'name', 'exercises']

class ExerciseSerializer(serializers.ModelSerializer):
    course_id = serializers.PrimaryKeyRelatedField(
          read_only=True
    )
    class Meta:
        model = Exercise
        fields = ['id','url', 'title', 'course_id']

view.py

class CourseViewSet(viewsets.ModelViewSet):
    queryset = Course.objects.all()
    serializer_class = CourseSerializer

class ExerciseViewSet(viewsets.ModelViewSet):
    queryset = Exercise.objects.all()
    serializer_class = ExerciseSerializer

urls.py

router = DefaultRouter()
router.register(r'courses', CourseViewSet)
router.register(r'exercises', ExerciseViewSet)

problem:

When I post data:

{
  "title": "Chapter 1",
  "course_id": 2
}

using postman, I get this error:

IntegrityError at /api/exercises/
null value in column "course_id" of relation "exercise" violates not-null constraint

image Here

Can any body help me?
Thanks.

[xiayulu]

I solve this by rewrite:

class ExerciseSerializer(serializers.ModelSerializer):
    course_id = serializers.PrimaryKeyRelatedField()
    class Meta:
        model = Exercise
        fields = ['id','url', 'title', 'course_id']

to

class ExerciseSerializer(serializers.ModelSerializer):
    course = serializers.PrimaryKeyRelatedField(
          queryset=Course.objects.all()
    )
    class Meta:
        model = Exercise
        fields = ['id','url', 'title', 'course']

which means writeable relation must provie a queryset.

See: https://stackoverflow.com/questions/17280007/retrieving-a-foreign-key-value-with-django-rest-framework-serializers

Now I can code happily again.

I hope this will help you.