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I have a ModelForm and I want to use my Model method for form validation process, I have tried form.save(commit=False) but it return a None object.
The period model have two attributes which are start and end time fields.

models.py

class Booking(models.Model):
 CATEGORY_CHOICES = (
 ('Web', 'Web Application'),
 ('Emb', 'Embedded Application')
 )
 user = models.ForeignKey(User, on_delete=models.CASCADE, related_name='bookings')
 room = models.ForeignKey(Room, on_delete=models.CASCADE, related_name='bookings')
 category = models.CharField(max_length=50, choices=CATEGORY_CHOICES, default='Web')
 date = models.DateField('Booking Date', db_index=True)
 start = models.ForeignKey(Period, on_delete=models.SET_NULL, related_name='start_at', null=True)
 end = models.ForeignKey(Period, on_delete=models.SET_NULL, related_name='end_at', null=True)
 used = models.BooleanField(default=False)

 objects = BookingManager()

 class Meta:
 ordering = ['-date']

 def get_start_time(self):
 return datetime.datetime.combine(self.date, self.start.start,
 tzinfo=timezone.get_current_timezone())

 def get_end_time(self):
 return datetime.datetime.combine(self.date, self.end.end,
 tzinfo=timezone.get_current_timezone())

 def is_occurring(self):
 now = timezone.localtime(timezone.now())
 return (now >= self.get_start_time()) and (now <= self.get_end_time())

 def extend_booking_time(self):
 next_end = self.end.next_period()

 if next_end is None:
 raise ValidationError("Invalid extending period")

 if not next_end.is_available(self.date, next_end):
 raise ValidationError("Overlapped extension")

 else:
 self.end = next_end
 self.save()

 def check_in(self):
 # Create record
 '''
 Log.objects.create(user=self.user.username, 
 room=self.room, booking= self.id)
 '''
 self.used = True
 self.save()

 def check_out(self, time):
 # Record Log
 #log = Log.objects.get(booking=self.id)
 #log.check_out = time
 #log.save()
 self.delete()

 def check_periods(self):
 return self.start.start < self.end.end

 def check_time(self):
 now = timezone.localtime(timezone.now())
 if now >= self.get_end_time():
 return False
 else:
 return True

 def check_overlap(self):
 start = self.get_start_time()
 end = self.get_end_time() 
 bookings = Booking.objects.filter(room=self.room, date=self.date)

 for booking in bookings:
 if (start < booking.get_end_time()) and (end > booking.get_start_time()):
 return False

 return True


 def clean(self):
 if not self.check_periods():
 raise ValidationError('Period error!')

 if not self.check_time():
 raise ValidationError('Time error!')

 if not self.check_overlap():
 raise ValidationError('Overlap error!')

forms.py

class BookingForm(forms.ModelForm):
 class Meta:
 model = Booking
 fields= ['room', 'date', 'start', 'end']

 def __init__(self, *args, **kwargs):
 initial_args = kwargs.get('initial', None)
 if initial_args:
 super(BookingForm, self).__init__(*args, **kwargs)
 self.fields['start'].widget = forms.TextInput()
 self.fields['date'].widget.attrs['readonly'] = True
 self.fields['start'].widget.attrs['readonly'] = True
 self.fields['room'].queryset = Room.objects.filter(
 id=initial_args['room'].id
 )
 self.fields['end'].queryset = Period.objects.get_available_periods(
 initial_args['room'],
 initial_args['date'],
 initial_args['start']
 )

 def clean_date(self):
 now = timezone.localtime(timezone.now()).date()
 date = self.cleaned_data['date']
 if date < now:
 raise ValidationError('Ngay dang ky khong hop le')

 return date

 def clean(self):
 pass

views.py

class BookingCreateView(LoginRequiredMixin, CreateView):
 login_url = 'login'
 form_class = BookingForm
 template_name = 'booking_add.html'
 success_url = reverse_lazy('booking_list')

 def get(self, request, *args, **kwargs):
 room = self.request.GET.get('room', None)
 date = self.request.GET.get('date', None)
 start = self.request.GET.get('start', None)
 if room is None or date is None or start is None:
 return redirect('select')
 else:
 room_object = get_object_or_404(Room, id=room)
 period = get_object_or_404(Period, number=start)
 date = datetime.datetime.strptime(
 date, '%d-%m-%Y'
 ).replace(tzinfo=timezone.get_current_timezone())

 if period.is_expired(date) or not period.is_available(room, date):
 return redirect(room_object.get_absolute_url())

 return super(BookingCreateView, self).get(request, *args, **kwargs)

 def get_initial(self):
 initial = super(BookingCreateView, self).get_initial()
 initial['date'] = datetime.datetime.strptime(
 self.request.GET.get('date'), '%d-%m-%Y'
 )
 initial['room'] = get_object_or_404(
 Room, id=self.request.GET.get('room')
 )
 initial['start'] = get_object_or_404(
 Period, number=self.request.GET.get('start')
 )
 return initial

 def form_valid(self, form):
 form.instance.user = self.request.user
 return super().form_valid(form)

template.

<main>
 <div class="reg-form">
 <form class="form" method="post" action="">
 % csrf_token %
 <label for="room">Phòng</label>
 form.room 
 <label for="date">Ngày</label>
 form.date 
 <label for="start">Ca bắt đầu</label>
 form.start 
 <label for="end">Ca kết thúc</label>
 form.end 
 <button type="submit">Đăng ký</button>
 <p> form.errors </p>
 </form>
 </div>
</main>

I want to use the method check_overlap in BookingForm validation process, what is the best way to do that?

The best way is pretty subjective but here's a way that isn't too involved.

In your model, call the method in clean. A ModelForm will call your model's clean method during the validation process (before you finish save).

Here are the relevant source portions:

https://github.com/django/django/blob/master/django/forms/models.py#L381
https://github.com/django/django/blob/master/django/forms/models.py#L381

Here's the model validation documentation: https://docs.djangoproject.com/en/dev/ref/models/instances/#validating-objects

Overriding clean on the form breaks the method resolution order so your model's clean won't get called the way this is written.

Hope this helps!