Django Many To Many Ordering - django

Question

I have two tables Subjectlist and Day. Subject list is m2m in Day. So my problem is I'm creating school timetable. So for each days different subjects to be shown, when i add subjects on each days the order of subject is same.
#Models.py
class SubjectList(models.Model):
subject_name = models.CharField(max_length=25)
def __str__(self):
return self.subject_name
class Day(models.Model):
day_name = models.CharField(max_length=15)
subject_name = models.ManyToManyField(SubjectList)
class_number = models.ForeignKey(AddClass, on_delete=models.CASCADE, null=True, blank=True)
start_time = models.TimeField(null=True, blank=True)
end_time = models.TimeField(null=True, blank=True)
def __str__(self):
return self.class_number.class_number
#Views.py
class TimeTableView(APIView):
def get(self, request, id):
class_number = AddClass.objects.get(id=id)
day = Day.objects.filter(class_number=class_number.id)
print(day)
serializer = DaySerializer(day, many=True)
return Response(serializer.data)
I want to do like this
Monday - English, maths, science, Social Science
Tuesady - Maths, Social Science, Englih, Math's
but i get like this
Monday - English, maths, science, Social Science
Tuesday- English, maths, science, Social Science
both are in same order even if add subjects in different order.

Answer 1

You can add more fields to the M2M table by declaring a model and assigning it to the M2M relation with the through parameter
class DaySubjectList(models.Model):
day = models.ForeignKey(Day, on_delete=models.CASCADE, related_name="day_subject_lists")
subject_list = models.ForeignKey(SubjectList, on_delete=models.CASCADE, related_name="day_subject_lists")
order = models.IntegerField(default=0)
class Day(models.Model):
day_name = models.CharField(max_length=15)
subject_name = models.ManyToManyField(SubjectList, through=DaySubjectList)
class_number = models.ForeignKey(AddClass, on_delete=models.CASCADE, null=True, blank=True)
start_time = models.TimeField(null=True, blank=True)
end_time = models.TimeField(null=True, blank=True)
def __str__(self):
return self.class_number.class_number
You can set the order field using the through_defaults parameter of the related manager
my_day.subject_name.add(my_subject, through_defaults={"position": my_position})
You can order using related_name
Day.objects.filter(class_number=class_number.id).prefetch_related(Prefetch("day_subject_lists", queryset=DaySubjectList.objects.order_by("position")))
Records will only be ordered if you access my_day.day_subject_lists.all() not my_day.subject_name.all()
You can store those records under another name if thats more convenient
Day.objects.filter(class_number=class_number.id).prefetch_related(Prefetch("day_subject_lists", queryset=DaySubjectList.objects.order_by("position"), to_attr="my_prefered_name"))
be aware that my_day.my_prefered_name is a list not a queryet so don't use .all() to access it.
Use you have any issue with your models referencing each other you can use string synthax
class DaySubjectList(models.Model):
day = models.ForeignKey("myapp.Day", on_delete=models.CASCADE, related_name="day_subject_lists")
subject_list = models.ForeignKey("myapp.SubjectList", on_delete=models.CASCADE, related_name="day_subject_lists")
order = models.IntegerField(default=0)

Answer 2

You need to specify an order for the m2m relation [Django-doc]:
class Day(models.Model):
# ...
subject_name = models.ManyToManyField(
SubjectList,
ordered=True
)
# ...
Now if you query a Day object, the related SubjectList objects will be ordered in the order you added these.
You can further alter the order with .move(...), or .reorder(...).
Note: You should rename the subject_name field to subjects, since it relates to multiple SubjectList objects. A ForeignKey or OneToOneField indeed often uses the _name suffix, since these relate to at most one other object, but a ManyToManyField does not have this limitation.
Note: You should not use ForeignKey(.., null=True)s. A foreign key represents a relation, and a relation can not be "NULL". You should make these fields not nullable, and make the related model have a nullable parent relation (with null=True), or make the field optional with blank=True.