class Product(models.Model):
name = models.CharField(max_length=50)
price = models.PositiveIntegerField(default=1)

def __str__(self):
    return self.name


class Order(models.Model):
id = models.AutoField(primary_key = True)
products = models.ManyToManyField(Product ,through='ProductOrder')

@property
def total(self):
    return self.productorder_set.aggregate(
        price_sum=Sum(F('quantity') * F('product__price'),
output_field=IntegerField()) )['price_sum']



class ProductOrder(models.Model):


product = models.ForeignKey(Product, on_delete=models.CASCADE ,
null=True,blank=True)
ordering = models.ForeignKey(Order, on_delete=models.CASCADE ,
blank=True)
quantity = models.IntegerField(default=1)

#to make a new instance from order,but doesnt make! before saving
ProductOrder
def create_order(sender, instance, **kwargs):

instance.ordering.save()


pre_save.connect(create_order,sender=ProductOrder)

class ProductOrdering(forms.ModelForm):
class Meta:
    model = ProductOrder
    fields = ['product','ordering','quantity']

class ProductOrderCreate(CreateView):
form_class = ProductOrdering
model = ProductOrder
template_name = 'create_product_order.html'
success_url = '/orders/'
def form_valid(self,form):
    form.instance.ordering = Order.objects.order_by('-pk')[0]

    return super(ProductOrderCreate,self).form_valid(form)