Django ORM: Get maximum value of a field with corresponding other fields values

Question

I have this Table (Counters):

cell_id	tftralacc	tfnscan	thtralacc	thnscan	date_time
13997	10	360	94	360	2022-02-22 00:00:00+01
13997	0	360	0	360	2022-02-22 01:00:00+01
13997	0	360	0	360	2022-02-22 02:00:00+01
13997	0	360	0	360	2022-02-22 03:00:00+01
13997	36	360	83	360	2022-02-22 04:00:00+01
13997	0	360	2	360	2022-02-22 05:00:00+01
13997	1	360	15	360	2022-02-22 06:00:00+01
13997	11	360	159	360	2022-02-22 07:00:00+01
13997	21	360	409	360	2022-02-22 08:00:00+01
13997	25	360	1282	360	2022-02-22 09:00:00+01
13997	20	360	1201	360	2022-02-22 10:00:00+01
13997	30	360	1381	360	2022-02-22 11:00:00+01
13997	42	360	924	360	2022-02-22 12:00:00+01
14000	1	360	36	360	2022-02-22 00:00:00+01
14000	0	360	0	360	2022-02-22 01:00:00+01
14000	1	360	0	360	2022-02-22 02:00:00+01
14000	0	360	2	360	2022-02-22 03:00:00+01
14000	0	360	0	360	2022-02-22 04:00:00+01
14000	0	360	12	360	2022-02-22 05:00:00+01
14000	3	360	4	360	2022-02-22 06:00:00+01
14000	24	360	123	360	2022-02-22 07:00:00+01
14000	31	360	374	360	2022-02-22 08:00:00+01
14000	18	360	620	360	2022-02-22 09:00:00+01
14000	38	360	1616	360	2022-02-22 10:00:00+01
14000	36	360	1410	360	2022-02-22 11:00:00+01
14000	24	360	957	360	2022-02-22 12:00:00+01

I want to get the specific date_time value of the maximum traffic (which is calculated based on the the fields tftralacc, tfnscan, thtralacc and thnscan) for every cell_id.

I've managed to get this maximum value for every cell_id by using the annotate() and group_by() functions of the Django's QuerySet API:

result = Counters.objects.filter(
    date_time__gte = date_start,
    date_time__lte = date_end
).annotate(
    # calculate the traffic for each row.
    traffic = Case(
        When(Q(tfnscan=0) or Q(thnscan=0), then=0),
        default = Round((F('tftralacc')*1.0/F('tfnscan')) + 
                        (F('thtralacc')*1.0/F('thnscan')), 2),
        output_field=FloatField()
    )
).order_by('cell_id').values(
    # Group by cell_id.
    'cell_id'
).order_by().annotate(
    # calculate the max traffic for the grouped Cells.
    max_traffic = Max('traffic')
)

The calculated traffic for every date_time is demonstrated here:

My code successfully returns the maximum traffic for every cell_id:

cell_id	max_traffic
13997	3.92
14000	4.59

But my goal is to get the Corresponding date_time value for every max value. like this:

cell_id	max_traffic	date_time
13997	3.92	2022-02-22 11:00:00+01
14000	4.59	2022-02-22 10:00:00+01

or

cell_id	date_time
13997	2022-02-22 11:00:00+01
14000	2022-02-22 10:00:00+01

Because that max value is just a mean to get the date_time and not the goal.

Note: There is this question that describes my problem, but its answer refers to a work-around solution, which is not possible with my problem. SO Question

Answer 1

Use models.Subquery with models.OuterRef to join on cell_id field. Then use queryset.annotate() to annotate the subquery with max_traffic. Finally, use queryset.filter() to select rows that have traffic equals to max_traffic and use .distinct() to remove duplicate rows.

counters_with_traffic = Counters.objects.filter(
    date_time__gte=date_start,
    date_time__lte=date_end
).annotate(
    # calculate the traffic for each row.
    traffic=Case(
        When(Q(tfnscan=0) | Q(thnscan=0), then=0),
        default=Round((F('tftralacc') * 1.0 / F('tfnscan')) +
                      (F('thtralacc') * 1.0 / F('thnscan')), 2),
        output_field=models.FloatField()
    )
)

counters_with_max_traffic = counters_with_traffic.order_by('cell_id').values(
    # Group by cell_id.
    'cell_id'
).order_by().annotate(
    # calculate the max traffic for the grouped Cells.
    max_traffic=Max('traffic'),
).filter(cell_id=models.OuterRef("cell_id")).values("max_traffic")

result = counters_with_traffic.annotate(
    max_traffic=models.Subquery(counters_with_max_traffic),
).filter(
    traffic=models.F("max_traffic")
).values(
    "cell_id", "max_traffic", "date_time"
).distinct("cell_id", "max_traffic")