Cannot assign "": "" must be a "User" instance

I'm making a project using the Django framework and is creating a website and is encountering this error:

Cannot assign "<User: >": "Booking.teacher" must be a "User" instance.

this is my model for User:

class User(AbstractUser):
    groups = models.ManyToManyField(
        'auth.Group',
        related_name='custom_user_set',  # Added related_name to avoid conflict
        blank=True,
    )
    user_permissions = models.ManyToManyField(
        'auth.Permission',
        related_name='custom_user_permissions_set',  # Added related_name to avoid conflict
        blank=True,
    )

    ROLE_CHOICES = [
        ('admin', 'Admin'),
        ('teacher', 'Teacher'),
        ('student', 'Student'),
    ]
    role = models.CharField(max_length=10, choices=ROLE_CHOICES)

this is my models.py for the booking:

class Booking(models.Model):
    room = models.ForeignKey(Room, on_delete=models.CASCADE)
    teacher = models.ForeignKey(User, on_delete=models.CASCADE, limit_choices_to={'role': 'teacher'})
    date = models.DateField()
    start_time = models.TimeField()
    end_time = models.TimeField()
    purpose = models.CharField(max_length=255, blank=True, null=True)

    class Meta:
        unique_together = ('room', 'date', 'start_time', 'end_time')

    def __str__(self):
        return f"{self.room.name} - {self.date} ({self.start_time} to {self.end_time})"

this is my forms.py:

class BookingForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.fields['teacher'].queryset = User.objects.filter(groups__name='teacher')  # Filter teachers

    class Meta:
        model = Booking
        fields = ['room', 'date', 'start_time', 'end_time', 'teacher']
        widgets = {
            'date': forms.DateInput(attrs={'type': 'date', 'class': 'form-control'}),
            'start_time': forms.TimeInput(attrs={'type': 'time', 'class': 'form-control'}),  # Updated to start_time
            'end_time': forms.TimeInput(attrs={'type': 'time', 'class': 'form-control'}),  # Added end_time
            'room': forms.TextInput(attrs={'class': 'form-control'}),
            'teacher': forms.Select(attrs={'class': 'form-control'}),
        }

    def clean(self):
        cleaned_data = super().clean()
        room = cleaned_data.get('room')
        date = cleaned_data.get('date')
        start_time = cleaned_data.get('start_time')
        end_time = cleaned_data.get('end_time')


        # Check for booking conflicts
        if Booking.objects.filter(room=room, date=date, start_time=start_time, end_time=end_time).exists():

            raise ValidationError('This room is already booked for the selected date and time.')

        return cleaned_data

and this is my views:

def booking_calendar(request):
    rooms = Room.objects.all()  # Fetch all rooms for the dropdown
    bookings = Booking.objects.all()

    if request.method == 'POST':
        form = BookingForm(request.POST)
        if form.is_valid():
            booking = form.save(commit=False)
            booking.teacher = request.user if request.user.groups.filter(name='teacher').exists() else None  # Assign the current user as the teacher if they are a teacher
            if booking.teacher is None:
                messages.error(request, 'You do not have permission to create a booking as a teacher.')  # Error message


            booking.save()
            messages.success(request, 'Booking created successfully!')
            return redirect('booking_calendar')
        else:
            messages.error(request, 'There was an error in your booking. Please check the form.')

    else:
        form = BookingForm()

    return render(request, 'booking_calendar.html', {'bookings': bookings, 'form': form, 'rooms': rooms})  # Pass rooms to the template

I tested to see if the form works and reflects into the database in making a booking but there's only this error Cannot assign "<User: mhar>": "Booking.teacher" must be a "User" instance. I'm a beginner in this and is my first time seeing this error. I'm stuck in this error pls help

Solution

You did not install the customized User model as the default user model, so request.user is a django.contrib.auth.models.User object, whereas teacher is a my_app.models.User model, and the two are thus in conflict.

You will need to set the AUTH_USER_MODEL setting ^[Django-doc] to your user model, so:

# settings.py

# …

AUTH_USER_MODEL = 'my_app.User'

with my_app the name of the app where you defined User.

if Booking.objects.filter(room=room, date=date, start_time=start_time, end_time=end_time).exists():

This does not check all possible conflicts. For example if there is a booking between 10AM and 13PM, and you want to book the room between 11AM and 14PM, this will not raise any validation error. You need to check with:

if (
    Booking.objects.exclude(pk=self.instance.pk)
    .filter(
        room=room, date=date, start_time__lt=end_time, end_time__gt=start_time
    )
    .exists()
):
    pass

Note: It is normally better to make use of the settings.AUTH_USER_MODEL ^[Django-doc] to refer to the user model, than to use the User model ^[Django-doc] directly. For more information you can see the referencing the User model section of the documentation ^[Django-doc].