#30246: Using an annotated field calculated with 
in aggregate results in ProgrammingError
               Reporter:  Jan        |          Owner:  nobody
  BaryƂa                             |
                   Type:  Bug        |         Status:  new
              Component:  Database   |        Version:  2.1
  layer (models, ORM)                |       Keywords:  query, aggregate,
               Severity:  Normal     |  extract, annotate
           Triage Stage:             |      Has patch:  0
  Unreviewed                         |
    Needs documentation:  0          |    Needs tests:  0
Patch needs improvement:  0          |  Easy pickings:  0
                  UI/UX:  0          |
 Aggregating most annotated fields works as expected, but if I put a
 DateTimeField through Extract during the annotate step, I get a
 ProgrammingError when trying to aggregate.

 class MyModel(models.Model):
     usage_time = models.DateTimeField()
     usage = models.FloatField()

 I would like to take the whole queryset, and calculate hourly usages. I
 figured using the django.db.models.functions.Extract transform would suit
 my needs well. This is the sample piece of code that, in a perfect
 scenario, would give me a dictionary filled with key value pairs, where
 key is the hour, and value is the sum of usages measured in that hour.

     hour_aggregates = {}
     for i in range(24):
         hour_aggregates['{}_{}'.format("am" if i < 12 else "pm", i)] =
 Sum("usage", filter=Q(hour=i))
     usages = MyModel.objects.annotate(hour=Extract("usage_time",

 Unfortunately, I get the following error:

 Traceback (most recent call last):
   File "/home/jan/project/env/lib/python3.6/site-
 packages/django/db/backends/utils.py", line 85, in _execute
     return self.cursor.execute(sql, params)
 psycopg2.ProgrammingError: column "__col2" does not exist
 LINE 1: ...CT "package_mymodel"."id" AS Col1, EXTRACT('hour' FROM "__col2"

 This occured to me while using Django 2.1.7. It doesn't work on 2.2b1, but
 I have tested this solution on Django 1.8 and it works, which is why I am
 filing this bug report. My Python version is 3.6.7 and I'm using
 PostgreSQL 10.6.

Ticket URL: <https://code.djangoproject.com/ticket/30246>
Django <https://code.djangoproject.com/>
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