Question

I have defined two models for retailers and items in their stock. Product is defined in another app. I defined two model methods to get and add stock items. Here is the relevant part of the code:

class Retailer(models.Model):
    name = models.CharField(max_length=100)
    @property
    def stock_items(self):
        return StockItem.objects.filter(retailer__id=F('id'))

    def add_stock_item(self, product_id):
        try:
            print "Checking if it's already in stock"
            self.stock_items.get(product__id=product_id)
            print "It's already in stock"
        except ObjectDoesNotExist:
            try:
                print "Adding item to stock"
                product = Product.objects.get(pk=product_id)
                StockItem.objects.create(retailer=self, product=product)
                print "Added to stock"
            except Product.DoesNotExist:
                print "Such product doesn't exist"
    def __unicode__(self):
        return self.name


StockItem(models.Model):
    retailer = models.ForeignKey(Retailer)
    product = models.ForeignKey(Product)
    def __unicode__(self):
        return "%s - %s %s" % (self.retailer, self.product.brand, self.product)

When I want to use these model methods, something weird happens. They stop working correctly after adding the first item (In these examples, product 1 is 16 GB iPhone and Product 2 is 32 GB iPhone).

First let’s add some products to our stock:

>>> r = Retailer.objects.get(pk=1)
>>> r.stock_items
[]
>>> r.add_stock_item(1)
Checking if it's already in stock
Adding item to stock
Added to stock
>>> r.add_stock_item(2)
Checking if it's already in stock
Adding item to stock
Added to stock

So far so good. Now let’s try adding products again to see how it handles errors:

>>> r.add_stock_item(1)
Checking if it's already in stock
It's already in stock
>>> r.add_stock_item(2)
Checking if it's already in stock
Adding item to stock
Added to stock

What? Why did it add product 2 again. It was supposed to show a message similar to product 1. Let’s see our stock:

>>> r.stock_items
[<StockItem: hh - Apple iPhone 4S 16GB>]

What happened to product 2? Did it fail to add it to database?

[<StockItem: hh - Apple iPhone 4S 16GB>, <StockItem: hh - Apple iPhone 4S 32GB>, <StockItem: hh - Apple iPhone 4S 32GB>]

Apparently not. It was added to database, but somehow our program fails to check it correctly. Only the first product added to stock is shown by calling r.stock_items. Restarting shell doesn’t change the situation either, so I guess the reason couldn’t be because of when the function is evaluated.

Why does this happen and how can I fix it?

Answer 1

Try to remove @property and change the method to this:

def stock_items(self):
    return self.stockitem_set.all()