python - Django "extra" query -

i have following model structure in models.py, items , offers:

class item(models.model):      status_choices = (         ('a', 'active'),         ('c', 'cancelled'),     )       status = models.charfield(max_length=11, choices=status_choices)     target_amount = models.positiveintegerfield()     ...  class offer(models.model)      status_choices = (         ('a', 'active'),         ('c', 'cancelled'),     )       status = models.charfield(max_length=11, choices=status_choices)     amount = models.positiveintegerfield()     item = models.foreignkey(item)     ... 

i want write query in views.py following each item:

1. the total of current (active) offers item
2. the percentage of target has been met offers item (i.e. [the total (1)]/target_amount * 100)

i have been trying achieve "extra" follows (using postgres):

items = item.objects.filter(status='a').extra(     select={         'total_amount' : 'select coalesce(sum(amount),0) myapp_offer myapp_offer.item_id = myapp_item.id , myapp_offer.status = \'a\'',         'percentage_met' : '(((select coalesce(sum(amount),0) myapp_offer myapp_offer.item_id = myapp_item.id , myapp_offer.status = \'a\') / target_amount) * 100)'     } ) 

now first select (total_amount) working correctly , returns expect, percentage_met 0. can me out why?

edit: should have mentioned want able order_by percentage_met

you don't need extra this. use aggregation:

from django.db.models import sum  items = item.objects.filter(status='a').annotate(total_amount=sum('offer__amount')) 

percentage_met per object, anyways, can method on model:

class item(models.model):     ...     @property     def percentage_met(self):         if hasattr(self, 'total_amount'):             return (self.total_amount / float(self.target_amount)) * 100         return none