Django模型中的order_by vs.aggregate

Question

Let's say I have a model called Orders, that has a field 'date' as a DateField with index_db=True.

given a date, I want to find the closest record in Orders to that date.

I can write:

prev_orders = Orders.objects.filter(date__lte=date).order_by('-date')
last_order = prev_orders[0]

or:

from django.db.models import Max
max_date = Orders.objects.filter(date__lte=date).aggregate(Max('date'))['date__max']
last_order = Orders.objects.get(date=max_date)

Which one is faster? are there any other reasons to prefer one for the other?

Answer 1

Orders.objects.filter(date__lte=date).order_by('-date')[0]

is obviously the faster one, in latter case you ORMing twice and calculating things more than you need..

First one is good enough..

Answer 2

prev_orders = Orders.objects.filter(date__lte=date).order_by('-date').first() 

first() swallow the resulting exception and return None if the queryset returns no objects. In case you are trying to get element by index [0] from empty queryset, Python should raise an error.