Code in views.py:

def add_ads(request):
    form_add_ads= AdvertisementForm
    if request.method== 'POST':
        form_add= AdvertisementForm(request.POST)
        if form_add.is_valid:
            form_add.save(commit=False)
            form_add.instance.author= request.user
            form_add.instance.slug= get_slug(form_add.instance.title)
            form_add.save()
            return redirect('/ads/')
        else:
            form_add_ads= AdvertisementForm()
    return render(request, 'ads/add_ads.html', {'form_add_ads': form_add_ads})```
Code in models.py:
```class Advertisement(models.Model):
    title= models.CharField(
        'Ad name (must be unique, character max 100)',
        max_length=100, unique=True)
    content= models.TextField('Ad description')
    author= models.ForeignKey(User, on_delete=models.CASCADE)
    publish= models.DateTimeField(auto_now_add=True)
    category= models.ForeignKey('Category', on_delete=models.CASCADE)
    slug= models.SlugField(max_length=100, unique=True)
    class Meta:
        verbose_name= 'Announcement'
        verbose_name_plural= 'Announcements'
        ordering= ['publish']
    def __str__(self):
        return self.title
    def get_absolute_url(self):
        return reverse('ads_detail', args=[self.publish.year,
                                           self.publish.month,
                                           self.publish.day,
                                           self.slug])

Code in forms.py:

class Meta:
        model= Advertisement
        fields= ('title', 'content', 'category')

The essence of the question: what should I add to the view, to check if the header is unique? Tried in different ways, mostly with crutches, nothing works. The view itself adds an article to the database based on the form, which in turn is generated from the model. At the moment, when trying to create ads with a non-unique title, a natural error occurs:

PS: I just started learning django (2 weeks), I will be grateful for any comments in the code.