Add "week_year" lookup to DateField/DateTimeField

[Sigurd Ljødal]

I found myself in the need to extract the ISO year/week-numbering year (as specified by ISO 8601) from a date/datetime field, e.g. to group values by week. This is easy with PostgreSQL:

from django.db.models.functions import Extract, ExtractWeek

SomeModel.objects.annotate(
    year=Extract('some_date', 'isoyear'),
    week=ExtractWeek('some_date'),
).values(
    'year', 'week',
).annotate(
    sum=Sum('some_field'),
)

but unfortunately extracting the week year like this does not work across databases. I've implemented an ExtractWeekYear class with support for all databases and will submit a pull request shortly.

[Sigurd Ljødal]

​PR

[Mariusz Felisiak]

As you mentioned there already exists ExtractWeek and week lookup (see #25240) and it works properly on all internal backends. Please clarify "this does not work across databases".

[Sigurd Ljødal]

Hey felixxm, looks like you've misunderstood what I have implemented, it's not the ExtractWeek class, but a cross database solution for Extract('some_date', 'isoyear'). I've updated the description a bit to clarify this.

The week-numbering year is different from the Julian year, as e.g. January 1st, 2017 is in week 52 of 2016.