python - Django - Time aggregates of DatetimeField across queryset -

(using django 1.11.2, python 2.7.10, mysql 5.7.18)

if imagine simple model:

class event(models.model):     happened_datetime = datetimefield()     value = integerfield() 

what elegant (and quickest) way run similar to:

res = event.objects.all().aggregate(     avg('happened_datetime') ) 

but able extract average time of day members of queryset. like:

res = event.objects.all().aggregate(     avgtimeofday('happened_datetime') ) 

would possible on db directly?, i.e., without running long loop client-side each queryset member?

edit:

there may solution, along lines, using raw sql:

select sec_to_time(avg(time_to_sec(extract(hour_second happened_datetime)))) event_event; 

performance-wise, runs in 0.015 second ~23k rows on laptop, not optimised, etc. assuming could yield accurate/correct results , since time secondary factor, using that?

so after little search , tries.. below seems work. comments on how improve (or hinting why wrong), welcome! :-)

res = event.objects.raw(''' select id, sec_to_time(avg(time_to_sec(extract(hour_second happened_datetime)))) average_time_of_day event_event happened_datetime between %s , %s;''', [start_datetime, end_datetime])  print res[0].__dict__ # {'average_time_of_day': datetime.time(18, 48, 10, 247700), '_state': <django.db.models.base.modelstate object @ 0x0445b370>, 'id': 9397l} 

now id returned of last object falling in datetime range clause. believe django inserts because of "invalidquery: raw query must include primary key".

quick explanation of sql series of function calls:

1. extract hh:mm:ss datetime fields
2. convert time values seconds via time_to_sec.
3. average seconds values
4. convert averaged seconds value time format (hh:mm:ss)

don't know why django insists on returning microseconds not relevant. (maybe local ms @ time object instantiated?)

performance note: seems extremely fast again haven't tested bit. insight kindly appreciated :)