How can i round an Sum in Django ojbect

My code:

verloningsoverzicht_cumulatief_dagen = 
Overzicht.objects.filter(periode=periode, jaar=jaar) 
            .values('testcontract', 'contract') 
            .annotate(aantal=Sum('aantal'))

Now lets say the .annotatie Sum is 6.3500000, i want it rounded to 6.35 The round method is not working, how can i solve this

.annotate(aantal=round(Sum('aantal'),2))

Answer

You can make use of a Round expression [Django-doc]:

from django.db.models.functions import Round

verloningsoverzicht_cumulatief_dagen = Overzicht.objects.filter(
    periode=periode, jaar=jaar
) .values('testcontract', 'contract').annotate(
    aantal=Round(Sum('aantal'), 2)
)

If you use or earlier, you can work with:

from django.db.models import DecimalField, ExpressionWrapper
from django.db.models.functions import Round

verloningsoverzicht_cumulatief_dagen = Overzicht.objects.filter(
    periode=periode, jaar=jaar
) .values('testcontract', 'contract').annotate(
    aantal=ExpressionWrapper(
        Round(Sum('aantal') * 100) / 100,
        output_field=DecimalField(max_digits=12, decimal_places=2)
    )
)