QuerySet silently allows querying with objects of wrong class

[Chris Wilson]

After a recent refactor, I thought our code was working correctly because the tests passed. Then I discovered that QuerySet allows you to pass objects of the wrong type in queries:

For example, this does not fail, and even returns some results:

Price.objects.filter(product=user.account_type)[0].pk

Even though Price.product is actually (now) a ForeignKey to Product, not AccountType. The correct code would be this:

Price.objects.filter(product__account_type=user.account_type)[0].pk

I think that QuerySet just extracts the object's PK without checking that it's an instance of the correct type.

I think it's not doing what is "obvious". I expect to get back Price objects whose product object is the same as the one I passed in, which is impossible if Price.product has a different class. Instead, it's silently rewritten my query into a less strict one, that only ensures that the FK is the same as the PK of the object I passed in, regardless of the type of that object.

Perhaps strictly it should return an empty set, because it's impossible for any Price objects to match the criteria that I provided, but I don't think that's very useful behaviour. Since this is a logic error in the application, I suggest throwing an exception to point it out instead.

[Luke Plant]

Duplicate of #16955