ManyToManyField is not saving in DB - django

Question

I have searched the existing answers on this platform related to this but it isn't helping me.
I save the Form but it is not saved in the DB.
views.py:
def student_list(request, pk) :
course = get_object_or_404(Course, pk=pk)
teacher = Instructor.objects.get(course_name=course)
attendance = Attendance.objects.create(course_name=course, instructor_name=teacher)
if request.method == "POST" :
form = PostForm(request.POST, instance=attendance)
if form.is_valid() :
attendance = form.save(commit=False)
attendance.published_date = timezone.now()
attendance.save()
return redirect('post_list')
else :
form = PostForm(instance=attendance)
return render(request, 'blog/student_list.html', {'course' : course, 'form' : form})
forms.py:
class PostForm(forms.ModelForm) :
class Meta :
model = Attendance
fields = ('student_name',)
def __init__(self, *args, **kwargs) :
super().__init__(*args, **kwargs)
if self.instance.pk :
self.fields['student_name'].queryset = Student.objects.filter(course_name=self.instance.course_name)
models.py:
class Attendance(models.Model) :
course_name = models.ForeignKey(Course, on_delete=models.SET_NULL, null=True)
student_name = models.ManyToManyField(Student)
instructor_name = models.ForeignKey(Instructor, on_delete=models.SET_NULL, null=True)
published_date = models.DateTimeField(blank=True, null=True)
student_list.html:
<form method="POST" class="post-form">{% csrf_token %}
{{ form.as_p }}
<button type="submit" class="save btn btn-default">Save</button>
</form>
Also, the fields course_name, instructor_name and published_date are saved in the DB. It is only the student_name that is not saved.

Answer 1

Well this is one of the reasons why using .save(commit=False) is indeed not a good idea, because a ModelForm will indeed save the ManyToManyField. This is done after the object itself is saved, since it first needs a primary key.
You can alter the instance wrapped in the form with:
def student_list(request, pk):
course = get_object_or_404(Course, pk=pk)
teacher = Instructor.objects.get(course_name=course)
if request.method == 'POST':
form = PostForm(request.POST)
if form.is_valid():
form.instance.course_name = course
form.instance.instructor_name = teacher
form.instance.published_date = timezone.now()
form.save()
return redirect('post_list')
else:
form = PostForm()
return render(request, 'blog/student_list.html', {'course': course, 'form': form})
That being said, if this is the standard behavior, you can just pass the timezone.now as default in your model:
from django.utils import timezone
class Attendance(models.Model):
course_name = models.ForeignKey(Course, on_delete=models.SET_NULL, null=True)
student_name = models.ManyToManyField(Student)
instructor_name = models.ForeignKey(Instructor, on_delete=models.SET_NULL, null=True)
published_date = models.DateTimeField(blank=True, null=True, default=timezone.now)
or even better, you can just use auto_now_add=True [Django-doc]:
class Attendance(models.Model):
course_name = models.ForeignKey(Course, on_delete=models.SET_NULL, null=True)
student_name = models.ManyToManyField(Student)
instructor_name = models.ForeignKey(Instructor, on_delete=models.SET_NULL, null=True)
published_date = models.DateTimeField(auto_now_add=True)
Note: A ForeignKey does not store the string representation (or name) of the
referenced object in the column, it stores the primary key of the record it
references in a column with an _id suffix to a ForeignKey field. Therefore
ForeignKeys usually do not end with a _name suffix. You might want to
consider renaming the course_name field to course.
Note: A GET request is not supposed to have side-effects, hence constructing
objects when a user makes a GET request, is not compliant with the HTTP
standard. Therefore it might be better to remove the creation of the Attendance
object at the database side.
EDIT: You can pass the course object to the PostForm to filter the queryset. For example:
class PostForm(forms.ModelForm) :
class Meta :
model = Attendance
fields = ('student_name',)
def __init__(self, *args, course=None, **kwargs) :
super().__init__(*args, **kwargs)
if self.instance.pk and not course:
course = self.instance.course_name
if course:
self.fields['student_name'].queryset = Student.objects.filter(course_name=course)
Then in the view, you can pass the Course object:
def student_list(request, pk) :
course = get_object_or_404(Course, pk=pk)
teacher = Instructor.objects.get(course_name=course)
if request.method == 'POST':
form = PostForm(request.POST, course=course)
if form.is_valid() :
form.instance.course_name = course
form.instance.instructor_name = teacher
form.instance.published_date = timezone.now()
form.save()
return redirect('post_list')
else :
form = PostForm(course=course)
return render(request, 'blog/student_list.html', {'course' : course, 'form' : form})