Question

I have the following code in my model, which determines if an Entry is billable:

class Entry(models.Model):
    [ .. ]
    @property
    def is_billable(self):
        return self.tags.filter(billable=False).count() == 0

Entry has an FK to Project. When determining the remaining budget on a Project, I loop through all the entries, and check if is_billable returns True:

@property
def remaining_budget(self):
    [ .. ]
    for entry in self.entry_set.all():
        if entry.is_billable:
            remaining_budget -= entry.minutes

Which is kind of heavy for the database, because it’ll fire a query for each Entry. I’m looking for a way to optimize this, tips and hints are welcome.

Answer 1

If you need to get rid of the count query then why not:

1. Add an is_billable field to your Entry model
2. Write signals based on the tag objects to set this field to True or False (if a new tag with is_billable == True is added, then set it True / if a tag is removed and no tags with is_billable == True remain, then set it False, etc)

Of course if your data can be changed through other apps or manually, this method should not be preferred.