Django: Query with F() into an object not behaving as expected

I am trying to navigate into the Price model to compare prices, but met with an unexpected result.

My model:

class ProfitableBooks(models.Model):
    price = models.ForeignKey('Price',primary_key=True)

In my vie开发者_C百科w:

foo = ProfitableBooks.objects.filter(price__buy__gte=F('price__sell'))

Producing this error:

'ProfitableBooks' object has no attribute 'sell' 

Is this your actual model or a simplification? I think the problem may lie in having a model whose only field is its primary key is a foreign key. If I try to parse that out, it seems to imply that it's essentially a field acting as a proxy for a queryset-- you could never have more profitable books than prices because of the nature of primary keys. It also would seem to mean that your elided books field must have no overlap in prices due to the implied uniqueness constraints.

If I understand correctly, you're trying to compare two values in another model: price.buy vs. price.sell, and you want to know if this unpictured Book model is profitable or not. While I'm not sure exactly how the F() object breaks down here, my intuition is that F() is intended to facilitate a kind of efficient querying and updating where you're comparing or adjusting a model value based on another value in the database. It may not be equipped to deal with a 'shell' model like this which has no fields except a joint primary/foreign key and a comparison of two values both external to the model from which the query is conducted (and also distinct from the Book model which has the identifying info about books, I presume).

The documentation says you can use a join in an F() object as long as you are filtering and not updating, and I assume your price model has a buy and sell field, so it seems to qualify. So I'm not 100% sure where this breaks down behind the scenes. But from a practical perspective, if you want to accomplish exactly the result implied here, you could just do a simple query on your price model, b/c again, there's no distinct data in the ProfitableBooks model (it only returns prices), and you're also implying that each price.buy and price.sell have exactly one corresponding book. So Price.objects.filter(buy__gte=F('sell')) gives the result you've requested in your snipped.

If you want to get results which are book objects, you should do a query like the one you've got here, but start from your Book model instead. You could put that query in a queryset manager called "profitable_books" or something, if you wanted to substantiate it in some way.