Exception when adding a product?

Question

Django

An exception occurs (NOT NULL constraint failed: market_product.fk_market_id), when I add a product to the store through the form. If null=True is set in the fk_market field, then there is no error, but the connection of the product with the store is not established and you need to do this through the admin panel

class Market(models.Model):
    category = models.ForeignKey(Category, on_delete=models.PROTECT)
    slug = models.SlugField(max_length=20, unique=True, db_index=True)
    name = models.CharField(max_length=30, unique=True, db_index=True)
    published = models.DateTimeField(default=timezone.now)
    seller = models.ForeignKey(Profile, on_delete=models.CASCADE)

    def save(self, *args, **kwargs):
        self.slug = slugify(self.name)
        super(Market, self).save(*args, **kwargs)

    def __str__(self):
        return self.name


class Product(models.Model):
    fk_market = models.ForeignKey(Market, on_delete=models.CASCADE)
    name_product = models.CharField(max_length=40, db_index=True)
    price = models.DecimalField(max_digits=10, decimal_places=2)
    image = models.ImageField(upload_to='products/%Y/%m/%d', null=True)
    description = models.TextField(max_length=200, blank=True)
    slug = models.SlugField(max_length=40, db_index=True)
    created = models.DateTimeField(auto_now_add=True)
    available = models.BooleanField(default=True)

    def save(self, *args, **kwargs):
        self.slug = slugify(self.name_product)
        super(Product, self).save(*args, **kwargs)

    def __str__(self):
        return self.name_product

Answer 1

It is possible to create in the form through obj = form.save(commit=False)then to add communication and again to save.

Answer 2

If someone has a similar problem, then you can link the models in this way

def market_detail(request, slug):
    market = get_object_or_404(Market, slug=slug)
    products = Product.objects.filter(fk_market__slug = slug)
    add = Market.objects.get(slug=slug)

    if request.method == 'POST':
        add_product = AddProductForm(request.POST)
        if add_product.is_valid():
            obj = add_product.save(commit=False)
            obj.fk_market = add
            obj.save()
            return redirect('/')
    else:
        add_product = AddProductForm()
    return render(request, 'market/market_detail.html', {'market': market,
                                                        'add_product': add_product,
                                                        'products': products})

PS But why the link is not created automatically and you need to do it in the view, I still do not understand, I would be grateful if you explain