How to update many to many Django REST? - django

Question

How would I go about updating a many to many relationship in Django REST framework?
Here is my model.
class SchoolTeacher(AbstractBase):
school = models.ForeignKey(School, on_delete=models.CASCADE,
related_name='teachers')
user = models.ForeignKey(User, on_delete=models.CASCADE,
related_name='teacher_at',)
subjects = models.ManyToManyField(SchoolSubject,
related_name='teachers')
Here is my serializer:
class SchoolTeacherSerializer(serializers.ModelSerializer):
....
def create(self, validated_data):
school_class = validated_data.get('school_class', None)
stream = validated_data.get('stream', None)
school_teacher_model_fields = [
f.name for f in SchoolTeacher._meta.get_fields()]
valid_teacher_data = {
key: validated_data[key]
for key in school_teacher_model_fields
if key in validated_data.keys()
}
subjects = valid_teacher_data.pop('subjects')
teacher = SchoolTeacher.objects.create(**valid_teacher_data)
for subject in subjects:
teacher.subjects.add(subject)
self.add_class_teacher(stream, school_class, teacher)
return teacher
def update(self, instance, validated_data):
subjects = validated_data.pop('subjects')
school_class = validated_data.get('school_class', None)
stream = validated_data.get('stream', None)
teacher = instance
for (key, value) in validated_data.items():
setattr(teacher, key, value)
for subject in subjects:
teacher.subjects.add(subject)
teacher.save()
return teacher
How do I achieve updating subjects? Currently, I can only add and not delete existing subjects.

Answer 1

Here is a workaround. I wish however someone would post a better answer. The idea here is to delete all subjects first then add them. However, since of course we cannot do teacher.subjects.all().delete(), we can do
for existing_subject in teacher.subjects.all():
teacher.subjects.remove(existing_subject)
for subject in subjects:
teacher.subjects.add(subject)

Answer 2

As we are talking about many-to-many, both objects need to be persisted before you can assign them to each other.
Once you have that, you can simply add one to the regarding collection of the other, like this (source: https://docs.djangoproject.com/en/2.1/topics/db/examples/many_to_many/):
a1 = Article(headline='Django lets you build Web apps easily')
a1.save()
p1 = Publication(title='The Python Journal')
p1.save()
a1.publications.add(p1)
Or, in one step:
new_publication = a1.publications.create(title='Highlights for Children')
I haven't found out how to express "contains" yet, but the next step is to remove the subjects that are not in the new list and add the ones that are in the new but not the old list. Probably more efficient with sorted lists or better hashes. But my python is not good enough for that yet.