django - queryset with extra method returns empty, count() says otherwise

Question

Here is a related manager I wrote:

class PortfolioItemManager(models.Manager):

    use_for_related_fields = True

    def extended(self): 

       return self.extra(select = {'current_price':'current_price', 'current_value':'current_price*quantity', 'gain':'current_price*quantity - cost'},
                      tables = ['pm_core_contract', ],
                      where = ['pm_core_contract.id = pm_core_portfolioitem.contract_id', ]
               )

Here are the results that stumps me:

In [10]: PortfolioItem.objects.extended()
Out[10]: []
In [11]: PortfolioItem.objects.extended().count()
Out[11]: 402

Result from count() is correct. What am I missing here?

EDIT: The generated SQL is correct and can be executed against the db directly.

EDIT2: The issue stems from the last 2 select arguments, which feature arithmetic operations.

Answer 1

I think I have just figured out the problem. Thanks go to Alex, whose comment has sparked the idea:

The PortfolioItem model has the properties current_value and current_gain, which I have been trying to replace with calculated SQL fields. It was my mistake to name one of the extra() method's select fields 'current_value' without removing the property, as that led to the model having two fields with the same name. When I did away with that overlapping, everything became OK. Lesson is learned.