Context I get different values for datetime field when I access them differently. I am sure there is some utc edge magic going on here.
(Pdb++)
Foo.objects.all().values_list('gated_out__occurred__date')[0][0]
datetime.date(2021, 9, 9)
(Pdb++) Foo.objects.all()[0].gated_out.occurred.date()
datetime.date(2021, 9, 10)
Edit: They have the same PK
Foo.objects.all().order_by("pk")[0].gated_out.occurred.date()
datetime.date(2021, 9, 10)
(Pdb++) Foo.objects.all().order_by("pk").values_list('gated_out__occurred__date')[0][0]
datetime.date(2021, 9, 9)
How do I fix/figure out what is happening?
Related
I'm finding that while nulls_last=True works, nulls_last=False doesn't. Example below is in a Django shell.
In [10]: [x.date for x in Model.objects.all().order_by(F('date').asc(nulls_last=True))]
Out[10]:
[datetime.datetime(2020, 3, 10, 16, 58, 7, 768401, tzinfo=<UTC>),
datetime.datetime(2020, 3, 10, 17, 4, 51, 601980, tzinfo=<UTC>),
None,
]
[ins] In [11]: [x.last_run_created_at for x in Model.objects.all().order_by(F('date').asc(nulls_last=False))]
Out[11]:
[datetime.datetime(2020, 3, 10, 16, 58, 7, 768401, tzinfo=<UTC>),
datetime.datetime(2020, 3, 10, 17, 4, 51, 601980, tzinfo=<UTC>),
None,
]
In [12]:
I've tried this with both desc() and asc().
The mistake is assuming that the opposite of nulls_last=True is nulls_last=False. It isn't.
nulls_last=True does the following to the query:
SELECT ... ORDER BY ... ASC NULLS LAST
Whereas nulls_last=False just means use the DB default:
SELECT ... ORDER BY ... ASC
What you want instead is to use nulls_first=True OR nulls_last=True to explicitly get the order you want.
This is mentioned in the docs, but perhaps not as explicitly as it could be:
Using F() to sort null values
Use F() and the nulls_first or
nulls_last keyword argument to Expression.asc() or desc() to control
the ordering of a field’s null values. By default, the ordering
depends on your database.
Take these models:
class Rocket(Model):
...
class Flight(Model):
rocket = ForeignKey(Rocket)
start_time = DateTimeField(...)
If I want to get start times of the latest flight for every rocket, that is simple:
>>> Flight.objects.values('rocket').annotate(max_start_time=Max('start_time'))
<QuerySet [
{'rocket': 3, 'max_start_time': datetime.datetime(2019, 6, 13, 6, 58, 46, 299013, tzinfo=<UTC>)},
{'rocket': 4, 'max_start_time': datetime.datetime(2019, 6, 13, 6, 59, 12, 759964, tzinfo=<UTC>)},
...]>
But what if instead of max_start_time I wanted to select IDs of those same Flights?
In other words, I want to get the ID of the latest Flight for every rocket.
What database backend are you using? If your database backend has support for DISTINCT ON this is most easily accomplished by:
Flight.objects.order_by("rocket", "-start_time").distinct("rocket").values("id", "rocket")
So to display a small bargraph using Django and Chart.js I constructed the following query on my model.
views.py
class BookingsView(TemplateView):
template_name = 'orders/bookings.html'
def get_context_data(self, **kwargs):
today = datetime.date.today()
seven_days = today + datetime.timedelta(days=7)
bookings = dict(Booking.objects.filter(start_date__range = [today, seven_days]) \
.order_by('start_date') \
.values_list('start_date') \
.annotate(Count('id')))
# Edit set default for missing dictonairy values
for dt in range(7):
bookings.setdefault(today+datetime.timedelta(dt), 0)
# Edit reorder the dictionary before using it in a template
context['bookings'] = OrderedDict(sorted(bookings.items()))
This led me to the following result;
# Edit; after setting the default on the dictionary and the reorder
{
datetime.date(2019, 8, 6): 12,
datetime.date(2019, 8, 7): 12,
datetime.date(2019, 8, 8): 0,
datetime.date(2019, 8, 9): 4,
datetime.date(2019, 8, 10): 7,
datetime.date(2019, 8, 11): 0,
datetime.date(2019, 8, 12): 7
}
To use the data in a chart I would like to add the missing start_dates into the dictionary but I'm not entirely sure how to do this.
So I want to update the dictionary with a value "0" for the 8th and 11th of August.
I tried to add the for statement but I got the error;
"'datetime.date' object is not iterable"
Like the error says, you can not iterate over a date object, so for start_date in seven_days will not work.
You can however use a for loop here like:
for dt in range(7):
bookings.setdefault(today+datetime.timedelta(dt), 0)
A dictionary has a .setdefault(..) function that allows you to set a value, given the key does not yet exists in the dicionary. This is thus shorter and more efficient than first checking if the key exists yourself since Python does not have to perform two lookups.
EDIT: Since python-3.7 dictionaries are ordered in insertion order (in the CPython version of python-3.6 that was already the case, but seen as an "implementation detail"). Since python-3.7, you can thus sort the dictionaries with:
bookings = dict(sorted(bookings.items()))
Prior to python-3.7, you can use an OrderedDict [Python-doc]:
from collections import OrderedDict
bookings = OrderedDict(sorted(bookings.items()))
There are two models a Price model and Service model. I'm trying to find the Services that have the most prices. All of this is initially filtered by a user input query (called the entry_query). The first line (1) get the price objects that the user queries (this works). Then, line 2 (2) returns a QS with the service_code and counts of prices a service has. Then, it brings on line (3) where it gives an error (see below).
(1) Price_objs = Price_filter.filter(entry_query)
(2) objs_filter=Price_objs.values_list('service__code').annotate(service_count=Count('service__code')).order_by('-service_count')
(3) serv_obj = Service.objects.filter(price__in = objs_filter).distinct()
Here is what line (2) outputs:
<QuerySet [('36430', 62), ('86003', 28), ('87149', 28), ('83516', 23), ('86317', 20), ('94640', 19), ('73502', 18), ('86658', 14), ('73721', 13), ('87070', 13), ('76942', 12), ('87081', 12), ('73560', 11), ('87798', 11), ('36415', 10), ('74177', 10), ('99211', 10), ('73100', 9), ('73221', 9), ('74176', 9), '...(remaining elements truncated)...']>
Here is the error that I get with line (3):
django.db.utils.OperationalError: sub-select returns 2 columns - expected 1
I have django model with DateField. I can get list of months of objects in such way:
months = [i.month for i in MyModel.objects.values_list('date', flat=True)]
and after delete duplicates I receive such list (example): [1, 2, 5, 6, 7, 9].
But if I have different years, I want to receive months in right date order. For example:
date1=31.08.2012, date2=31.12.2012, date3=05.05.2013.
So I want to receive not [5, 8, 12] but [8, 12, 5].
How can I do it?
You're trying to get the months in order of when they first appear chronologically?
list(set(date.month for date in MyModel.objects.order_by("date").values_list('date', flat=True)))
Sorting by year is the same as sorting by date. Yay!
The only way to do it would be to add the year in too:
dates = [(i.year, i.month) for i in MyModel.objects.values_list('date', flat=True)]
That would return this list (once duplicates are removed and sorted):
[(2012, 8), (2012, 12), (2013, 5)]
If you wanted later, you could then get just the months by:
>>> [x[1] for x in dates]
[8, 12, 5]
But note that there may well be duplicates in that list too (August In both 2012 and 2013 would come out as 8, for example), and you wouldn't necessarily know where the list changes from one year to the next.