Django and SQLite: DurationField arithmetics
I have the following model
class MyModel(models. Model):
start = models. TimeField()
finish = models. TimeField()
penalty = models. IntegerField(blank=True, default=0) # penalty in minutes
and the following query set comments in Administration
def get_queryset(self, request):
qs = super().get_queryset(request)
qs = qs.annotate(
total_time=ExpressionWrapper(
F("finish") - F("start") + F("penalty") * 60,
output_field=DurationField()
)
)
return qs
When I check the comment value of the penalty !=
0 line, I get something like datetime.timedelta(0, 1800, 1800) (in this case, 30
minutes for penalty).
However, I expect time increments like datetime.timedelta(0, 3600).
Can someone explain?
I tried it too
def get_queryset(self, request):
qs = super().get_queryset(request)
qs = qs.annotate(
duration=ExpressionWrapper(
F("finish") - F("start"),
output_field=DurationField()
)
)
qs = qs.annotate(
total_time=ExpressionWrapper(
F("duration") + F("route_shortening_penalty") * 60,
output_field=DurationField()
)
)
return qs
Even
def get_queryset(self, request):
qs = super().get_queryset(request)
qs = qs.annotate(
duration=ExpressionWrapper(
F("finish") - F("start"),
output_field=DurationField()
),
penalty_duration=ExpressionWrapper(
F("penalty") * 60,
output_field=DurationField()
)
)
qs = qs.annotate(
total_time=ExpressionWrapper(
F("duration") + F("penalty_duration"),
output_field=DurationField()
)
)
return qs
Both lead to
Traceback (most recent call last):
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/core/handlers/exception.py", line 34, in inner
response = get_response(request)
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/core/handlers/base.py", line 126, in _get_response
response = self.process_exception_by_middleware(e, request)
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/core/handlers/base.py", line 124, in _get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/contrib/admin/options.py", line 604, in wrapper
return self.admin_site.admin_view(view)(*args, **kwargs)
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/utils/decorators.py", line 142, in _wrapped_view
response = view_func(request, *args, **kwargs)
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/views/decorators/cache.py", line 44, in _wrapped_view_func
response = view_func(request, *args, **kwargs)
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/contrib/admin/sites.py", line 223, in inner
return view(request, *args, **kwargs)
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/utils/decorators.py", line 45, in _wrapper
return bound_method(*args, **kwargs)
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/utils/decorators.py", line 142, in _wrapped_view
response = view_func(request, *args, **kwargs)
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/contrib/admin/options.py", line 1793, in changelist_view
'selection_note': _('0 of %(cnt)s selected') % {'cnt': len(cl.result_list)},
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/db/models/query.py", line 250, in __len__
self._fetch_all()
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/db/models/query.py", line 1186, in _fetch_all
self._result_cache = list(self._iterable_class(self))
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/db/models/query.py", line 63, in __iter__
for row in compiler.results_iter(results):
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/db/models/sql/compiler.py", line 1009, in apply_converters
value = converter(value, expression, connection)
File "/home/dm/.virtualenvs/test/lib/python3.6/site-packages/django/db/backends/base/operations.py", line 573, in convert_durationfield_value
return datetime.timedelta(0, 0, value)
TypeError: unsupported type for timedelta microseconds component: str
I’m using Python 3.6.7
, Django==2.1.7
, and SQLite version 3022000
.
Solution
If you get datetime.timedelta(0,
1800, 1800) instead of the correct datetime.timedelta(0, 3600)
then you’re obviously pretty much there.
I assume that 1800
seconds is the difference between F(“finish”) and F(“start"),
while 1800 microseconds comes from F("penalty") * 60
.
Multiply by 60*1000*1000
instead of 60
to convert penalty
from minutes to microseconds:
total_time=ExpressionWrapper(
F("finish") - F("start") + F("penalty") * 60000000,
output_field=DurationField()
)