Mock CTO interview: two customers buy the last item at the same time

# ======================================================================
# Q: CTO: "Two customers buy the last product at the same time. What happens in
# your system?"
#
# You: "If the code reads the stock in Python, checks it and then saves, we get
# a lost update and overselling. I let the database guarantee it with a
# conditional atomic UPDATE: if 0 rows are affected, the product is sold out and
# the second user sees a friendly message. As a safety net I have a
# CheckConstraint so the stock can never become negative. If the flow is more
# complex (multiple products, coupons), I lock with select_for_update in a fixed
# order." Chapter 4.
# ======================================================================

def reserve(product_id, qty):
    updated = (Product.objects
               .filter(pk=product_id, stock__gte=qty)
               .update(stock=F('stock') - qty))
    if not updated:
        raise OutOfStock(product_id)

class Product(models.Model):
    stock = models.IntegerField()
    class Meta:
        constraints = [models.CheckConstraint(
            condition=Q(stock__gte=0), name='product_stock_non_negative')]