Wrong assertion in resolve_aggregate in django.db.models.sql.query

[Ghislain Leveque]

In the resolve_aggregate method in the django.db.models.sql.query module :

297 	        elif aggregate.is_computed:
298 	            # Any computed aggregate (e.g., avg) returns a float
299 	            return float(value)

The assertion that "any computed aggregate like AVG returns a float" is wrong. In particular, I use Postgresql/pyscopg2 and postgresql returns an interval (datetime.timedelta from the python point of vue) when the aggregated column is of type interval.

I made a quick and dirty patch attached to the ticket

[Ghislain Leveque]

This patch add support for the timedelta aggregation

[Russell Keith-Magee]

I'll mark this as accepted, but:

1. I'm not a fan of the isinstance call. Unless you can prove that timedelta is the *only* type that needs this sort of raw handling, then your patch should be trying to identify the underlying condition that means that a raw database value should be passed through unmodified.
2. I'd be very surprised if timedeltas are handled consistently across databases. Time handling varies wildly between backends. However, it's impossible to validate if this is a problem this because...
3. Your patch doesn't contain tests. No patch is complete until there are tests to validate the new behavior.

[Collin Anderson]

Here is a similar issue with resolve_aggregate, although not 100% related. I would like to be able to annotate by a CharField instead of an aggregate. Currently I need to monkey patch resolve_aggregate on the fly to get it to return unicode instead of float. Please see my example code here:

​http://gist.github.com/333355

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Tim Graham]

The code in question in gone as of Django 1.8 and the expressions refactor.