QuerySet.extra(select=...) is silently dropped with aggregations

[jdelic]

Here is a minimal example:

from django.db import models

class Service(models.Model):
    name = models.TextField()

class ServiceAction(models.Model):
    performed = models.DateTimeField(auto_now_add=True)
    service = models.ForeignKey(Service)

Then add some models:

>>> from dbtest.models import Service, ServiceAction
>>> from django.db.models import Min, Max
>>> s1 = Service.objects.create(name='test1')
>>> s2 = Service.objects.create(name='test2')
>>> ServiceAction.objects.create(service_id=s1.id)
>>> ServiceAction.objects.create(service_id=s1.id)
>>> ServiceAction.objects.create(service_id=s1.id)
>>> ServiceAction.objects.create(service_id=s2.id)
>>> ServiceAction.objects.create(service_id=s2.id)
>>> ServiceAction.objects.create(service_id=s2.id)

...and query them using aggregations:

>>> qs = ServiceAction.objects.extra(select={'period': '(MAX("dbtest_serviceaction"."performed")-MIN("dbtest_serviceaction"."performed"))'}).values('service_id').annotate(first_action=Min('performed'), last_action=Max('performed'))
>>> print(qs.query)
SELECT "dbtest_serviceaction"."service_id", MAX("dbtest_serviceaction"."performed") AS "last_action", MIN("dbtest_serviceaction"."performed") AS "first_action" FROM "dbtest_serviceaction" GROUP BY "dbtest_serviceaction"."service_id"

As you can see the .extra(select=...) has been dropped from the query. I assume this happens on Django 1.6.x, 1.7.x and 1.8.x, but I have only tested 1.6.11 and 1.8.

The expected outcome would have been:

SELECT "dbtest_serviceaction"."service_id", MAX("dbtest_serviceaction"."performed") AS "last_action", MIN("dbtest_serviceaction"."performed") AS "first_action",  MAX("dbtest_serviceaction"."performed")-MIN("dbtest_serviceaction"."performed") as "period" FROM "dbtest_serviceaction" GROUP BY "dbtest_serviceaction"."service_id"

In Django 1.8 this exact query can be performed with the ORM itself through the use of chained aggregations:

>>> qs = ServiceAction.objects.values('service_id').annotate(first_action=Min('performed'), last_action=Max('performed'), period=Max('performed')-Min('performed'))
>>> print(qs.query)
SELECT "dbtest_serviceaction"."service_id", (MAX("dbtest_serviceaction"."performed") - MIN("dbtest_serviceaction"."performed")) AS "period", MAX("dbtest_serviceaction"."performed") AS "last_action", MIN("dbtest_serviceaction"."performed") AS "first_action" FROM "dbtest_serviceaction" GROUP BY "dbtest_serviceaction"."service_id"