class Product(models.Model):
    name = models.CharField(max_length=255)
    price = models.DecimalField(decimal_places=2, max_digits=12)
    store = models.ForeignKey(Store)

class Variation(models.Model):
    product = models.ForeignKey(Product)
    title = models.CharField(max_length=255)
    active = models.BooleanField(default=True)
    stock = models.PositiveIntegerField(null=True, blank=True)
    deleted = models.BooleanField(default=False)

class Orderitem(models.Model):
    variation = models.ForeignKey(Variation)
    quantity = models.PositiveIntegerField()  # I want to display the sum of this
    order = models.ForeignKey(Order)

class Product(models.Model):
    ...

    def get_total_order(self):
         return ...

{{ product.get_total_order }}

def your_view(request, ...):
     return render(request, template_name...html, {'total_order': xxx})

{{ total_order }}

{{ product|product_total_order }}