I want to annotate the difference between a DateTime and an Integer (seconds) on a legacy MySQL DB (so there is no chance to change the IntegerField to a DurationField) with Django 1.11
Report(Model):
# time the report was sent
time = DateTimeField()
# seconds since the last reboot
uptime = IntegerField()
MyModel.objects.all().annotate(
last_report=Max(time),
last_reboot=ExpressionWrapper(F('last_report') - F('uptime')), output_field=DateTimeField())
)
This would work, if uptime
was a DurationField(), but won't work with an integer. So I tried converting the seconds to a timedelta
last_reboot=ExpressionWrapper(
F('last_report') - F(timezone.timedelta(seconds=1)*F('uptime')),
output_field=DateTimeField()
)
which gives me
AttributeError at ...
'CombinedExpression' object has no attribute 'split'
Is there a way to calculate with a DateTimeField() and a IntegerField() in a query expression?
I found a solution: Convert the integer to a duration using the ExpressionWrapper
. As I am getting seconds and a DurationField()
is Microseconds, I have to multiply them by 1,000,000.
last_reboot=ExpressionWrapper(
F('last_report') - ExpressionWrapper(
F('uptime') * 1000000,
output_field=DurationField()
),
output_field=DateTimeField()
)
Previously, I did something like below...might be useful for others.
.annotate(timestamp=RawSQL('DATE(milisecond_datetimestamp, \'unixepoch\')',[]))
and another query where I had to convert the date integer field to hour
.annotate(hour=Round(F('milisecond_datetimestamp') % (3600 * 24) /3600))
and for Round
you have to define SQL function like below
class Round(Func):
function= 'ROUND'
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