Django - Can you use property as the field in an aggregation function?

Question

I know the short answer because I tried it. Is there any way to accomplish this though (even if only on account of a hack)?

class Ticket(models.Model):
    account = modelfields.AccountField()
    uuid = models.CharField(max_length=36, unique=True)
    created = models.DateTimeField(auto_now_add=True)

    class Meta:
        ordering = ['created']

    @property
    def repair_cost(self):
        # cost is a @property of LineItem(models.Model)
        return self.lineitem_set.aggregate(models.Sum('cost'))

You really need to show the LineItem model also. – Jason Christa Jun 18 '10 at 00:31 — Jason Christa, Jun 18 '10 at 00:31

score 28 · Accepted Answer · answered Jun 18 '10 at 00:59

28

No. Anything that goes through a built-in manager has to be a real field, since they only touch the database. In order to work with a property they'd have to turn every record in the table into a model, then filter through them in Python.

answered Jun 18 '10 at 00:59

Ignacio Vazquez-Abrams

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thanks. I thought that might be the case. I pictured `get all lineitemss and then call their cost property` but that would be so expensive :( – orokusaki Jun 18 '10 at 14:56

score 5 · Answer 2 · answered Jun 20 '16 at 07:17

5

I have a similar scenario and want exactly the same feature. I solved it trivially with the following line:

...
return sum(lt.cost for lt in self.lineitem_set)

answered Jun 20 '16 at 07:17

Fish Monitor

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But it's slow, too slow, compared to django ORM – mr L Apr 12 '19 at 12:34
@abinax what would you suggest then? – ataraxis Dec 11 '20 at 01:18
This hits the database as a seperate query for every object in the queryset. – William Feb 12 '21 at 17:58

Django - Can you use property as the field in an aggregation function?

2 Answers2

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