Django CreateView show error not working

Question

how i can prevent the Repetition of renting a car for the same period the car was already rented or reserved for ex : 01/01/2017 to 01/03/2017 if i trie to rent this car for a period between this dates " 01/01/2017 to 01/03/2017" it should give me a error like the validation error in the right of the field

screen shot for my rental car app

models.py

class Car(models.Model):

    imatriculation = models.CharField(max_length=100, primary_key=True)
    marque = models.CharField(max_length=100)
    couleur = models.CharField(max_length=100)

class Contra(models.Model):

    car = models.ForeignKey(Car, on_delete=models.CASCADE)
    date_debut = models.DateField(null=True ,blank=True)
    heure_debut = models.TimeField(null=True ,blank=True)
    date_retour = models.DateField(null=True ,blank=True)
    heure_retour = models.TimeField(null=True ,blank=True)

forms.py

class CarForm(forms.ModelForm):

    class Meta:
        model = Car
        fields = ('imatriculation', 'marque',  'couleur')       


class ContraForm(forms.ModelForm):

    date_debut = forms.DateField(widget = AdminDateWidget())
    heure_debut = forms.TimeField(widget = AdminTimeWidget())
    date_retour = forms.DateField(widget = AdminDateWidget())
    heure_retour = forms.TimeField(widget = AdminTimeWidget())

    class Meta:
        model = Contra
        fields = ('car','date_debut','heure_debut',  
                 'date_retour','heure_retour')

views.py

class ContraCreate(CreateView):
    template_name = 'rentcar/contra_update.html'
    form_class = ContraForm
    model = Contra

    def form_valid(self, form):
                now_date = date.today()
                now_time = datetime.now().strftime('%H:%M:%S')
                x = Contra.objects.filter(location=self.request.user,
                    date_debut__lte=now_date, date_retour__gte=now_date)
                c = form.instance.car
                for e in x:
                    if c.imatriculation ==  e.car.imatriculation :
                    print ("Leased car")
                    ValidationError(_('Invalid value'))
                    return super(ContraCreate, self).form_invalid(form)
                return super(ContraCreate, self).form_valid(form) 

i find a solution using clean

def clean(self):
    super(ContratForm, self).clean()
    form_car          = self.cleaned_data.get('car')    
    form_date_debut   = self.cleaned_data.get('date_debut')
    form_date_retour  = self.cleaned_data.get('date_retour')
    form_heure_debut  = self.cleaned_data.get('heure_debut')
    form_heure_retour = self.cleaned_data.get('heure_retour')
    h                 = form_car.imatriculation

    x1  = Contrat.objects.filter(location=self.user,car=h ,date_debut__lte=form_date_debut,date_retour__gte=form_date_retour)
    x2  = Contrat.objects.filter(location=self.user,car=h ,date_debut__gt=form_date_debut,date_debut__lt=form_date_retour,date_retour__gte=form_date_retour)
    x25 = Contrat.objects.filter(location=self.user,car=h ,date_debut__gt=form_date_debut,date_debut=form_date_retour,heure_debut__lt=form_heure_retour)
    x3  = Contrat.objects.filter(location=self.user,car=h ,date_debut__lte=form_date_debut,date_retour__gt=form_date_debut,date_retour__lt=form_date_retour)
    x35 = Contrat.objects.filter(location=self.user,car=h ,date_debut__lte=form_date_debut,date_retour=form_date_debut,heure_retour__gt=form_heure_debut)
    x4  = Contrat.objects.filter(location=self.user,car=h ,date_debut__gt=form_date_debut,date_retour__lt=form_date_retour)


    if x1:
        for k in x1:
            self._errors['car']          = "Deja reserve .contrat %s" %(k.id)
            self._errors['date_debut']   = k.date_debut.strftime('%d/%m/%Y')
            self._errors['date_retour']  = k.date_retour.strftime('%d/%m/%Y')
    elif x2:
        for k in x2:
            self._errors['car']          = "Deja reserve .contrat %s" %(k.id)
            self._errors['date_retour']  = ("<= %s" %(k.date_debut).strftime('%d/%m/%Y'))
    elif x25:
        for k in x25:
            self._errors['car']          = "Changer l'heur retour"
            self._errors['date_retour']  = ("= Date depart contrat %s" %(k.id))
            self._errors['heure_retour'] = ("< %s" %(k.heure_debut))
    elif x3:
        for k in x35:
            self._errors['car']          = "Deja reserve .contrat %s" %(k.id)
            self._errors['date_debut']   = (">= %s" %(k.date_retour).strftime('%d/%m/%Y'))
    elif x35:
        for k in x35:
            self._errors['car']          = "Changer l'heur de depart"
            self._errors['date_debut']   = ("= Date debut contrat %s" %(k.id))
            self._errors['heure_debut']  = ("> %s" %(k.heure_retour))
    elif x4:
        for k in x4:
            self._errors['car']          = "Deja reserve .contrat %s" %(k.id)
            self._errors['date_debut']   = k.date_debut.strftime('%d/%m/%Y')
            self._errors['date_retour']  = k.date_retour.strftime('%d/%m/%Y')

    return self.cleaned_data

Answer 1

By the time form_valid in the view is called, Django has already decided that the form is valid. Check out what the docs say about how validation works.

The extra validation you want to do against data in the database already can be done by overriding the clean stage of validation in the form.

A solution based on your above code would look something like:

def clean(self):
    '''This goes in ContraForm.py'''
    cleaned_data = super(ContraForm, self).clean()
    now_date = date.today()
    now_time = datetime.now().strftime('%H:%M:%S')
    x = Contra.objects.filter(location=self.request.user,
                date_debut__lte=now_date, date_retour__gte=now_date)
    c = form.instance.car
    for e in x:
        if c.imatriculation ==  e.car.imatriculation :
            raise ValidationError('Invalid debut date. Car already reserved.')
    return cleaned_data