Django ORM select, concat, extract from data and order by - django

Question

I'm having trouble performing a simple transformation with the django orm.
Desired outcome should look like this:
2018-08
2018-07
2018-06
...
And is created with this sql:
select
distinct
strftime('%Y',a."Buchung") || "-" ||
strftime('%m',a."Buchung") as YearMonth
from
hhdata_transaktion a
order by
1 desc
I need it for a ModelChoiceField as queryset, so I'm bound to the ORM here?
My try
from django.db.models.functions import TruncMonth, TruncYear
Transaktion.objects
.annotate(year=TruncYear('Buchung'),
month=TruncMonth('Buchung'))
.distinct()
.order_by('-year', '-month')
.values('year','month')
returns:
<QuerySet [{'year': datetime.date(2018, 1, 1), 'month': datetime.date(2018, 8, 1)}, {'year': datetime.date(2018, 1, 1), 'month': datetime.date(2018, 7, 1)}, {'year': datetime.date(2018, 1, 1), 'month': datetime.date(2018, 6, 1)}, {'year': datetime.date(2018, 1, 1), 'month': datetime.date(2018, 5, 1)}, {'year': datetime.date(2018, 1, 1), 'month': datetime.date(2018, 4, 1)}, {'year': datetime.date(2018, 1, 1), 'month': datetime.date(2018, 3, 1)}, {'year': datetime.date(2018, 1, 1), 'month': datetime.date(2018, 2, 1)}, {'year': datetime.date(2018, 1, 1), 'month': datetime.date(2018, 1, 1)}, {'year': datetime.date(2017, 1, 1), 'month': datetime.date(2017, 12, 1)}, {'year': datetime.date(2017, 1, 1), 'month': datetime.date(2017, 11, 1)}, {'year': datetime.date(2017, 1, 1), 'month': datetime.date(2017, 10, 1)}, {'year': datetime.date(2017, 1, 1), 'month': datetime.date(2017, 9, 1)}, {'year': datetime.date(2017, 1, 1), 'month': datetime.date(2017, 8, 1)}]>
I have the feeling I'm miles away from the desired result..

Answer 1

If you want to obtain the year or month, you can use ExtractYear [Django-doc] and ExtractMonth [Django-doc] respectively. Truncating will give you the start of the year or month.
So we can rewrite the query to:
from django.db.models.functions import ExtractMonth, ExtractYear
qs = Transaktion.objects.annotate(
year=ExtractYear('Buchung'),
month=ExtractMonth('Buchung')
).order_by('-year', '-month').values('year','month').distinct()
Although it is possible to do the processing at SQL level, I think it will make work more complex. For example if you concatenate the numbers in SQL, it will probably require some work to get leading zeros for months (less than 10). Furthermore it is likely that the query contains "SQL dialect"-specific features making it less portable.
Therefore I suggest to do the post processing at the Django/Python level. For exampe with:
from django.db.models.functions import ExtractMonth, ExtractYear
class MyForm(forms.Form):
my_choice_field = forms.ChoiceField()
# ...
def __init__(self, *args, **kwargs):
super(MyForm, self).__init__(*args, **kwargs)
qs = Transaktion.objects.annotate(
year=ExtractYear('Buchung'),
month=ExtractMonth('Buchung')
).order_by('-year', '-month').values('year','month').distinct()
self.fields['my_choice_field'].choices = [
(row['year']*100+row['month'], '{}-{:02d}'.format(row['year'], row['month'])
for row in qs
]
Here we thus generate a list of 2-tuples where the first element is some sort of number we use to identify the choice (I here multiplied the year by 100, such that 201804 is april 2018). The second element of the tuple is the string that determines the format.

Answer 2

If you want a list of strings like 2018-06, something like that should work:
[ '%i-%02i' % (x.Buchung.year, x.Buchung.month) for x in Transaktion.objects.order_by(-Buchung) ]