django listview order by model property

Question

I have a model called Lap with a property calculated using data from the table

class Lap(models.Model):
    team=models.ForeignKey('Team', models.DO_NOTHING)
    timestamp=models.IntegerField()
    num=models.IntegerField()
    
    @property
    def laptime(self):
        endtime=self.timestamp
        starttime=Lap.objects.get(team=self.team, num=self.num-1).timestamp)
        return time.timedelta(seconds=endtime-starttime)

I am trying to create a listview for the model

class FastestLap(ListView):
    model=Lap
    template_name='fastestlaps.html'
    context_object_name='laps'

I want to order the list view by the laptime property.

sorting by a column can be done using the ordering variable or by creating a get_queryset method and doing queryset.order_by(fieldname) in that method but I cant find a way to order by the property. How do I order by laptime?

Asked By: codingcat3423

||

Source

Answer 1

Use Window, F and Lag functions and annotate so:

from django.db.models import Window, F
from django.db.models.functions import Lag

# from .models import Lap
# from django.views.generic import ListView

class FastestLap(ListView):
    queryset = Lap.objects.annotate(
        starttime=Window(
            expression=Lag('timestamp', default=0),
            partition_by=['team'],
            order_by=F('num').asc(),
        )
    ).annotate(
        laptime=F('timestamp') - F('starttime')
    ).order_by('laptime')
    template_name = 'fastestlaps.html'
    context_object_name = 'laps'

References:

Answered By: anjaneyulubatta505

django listview order by model property

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