How can I make a Django update with a conditional case?

Question

I would like to use Django to update a field to a different value depending on its current value, but I haven't figured out how to do it without doing 2 separate update statements.

Here's an example of what I'd like to do:

now = timezone.now()
data = MyData.objects.get(pk=dataID)
if data.targetTime < now:
    data.targetTime = now + timedelta(days=XX)
else:
    data.targetTime = data.targetTime + timedelta(days=XX)
data.save()

Now, I'd like to use an update() statement to avoid overwriting other fields on my data, but I don't know how to do it in a single update(). I tried some code like this, but the second update didn't use the up to date time (I ended up with a field equal to the current time) :

# Update the time to the current time
now = timezone.now()
MyData.objects.filter(pk=dataID).filter(targetTime__lt=now).update(targetTime=now)
# Then add the additional time
MyData.objects.filter(pk=dataID).update(targetTime=F('targetTime') + timedelta(days=XX))

Is there a way I can reduce this to a single update() statement? Something similar to the SQL CASE statement?

Answer 1

You need to use conditional expressions, like this

from django.db.models import Case, When, F

object = MyData.objects.get(pk=dataID)
now = timezone.now()
object.targetTime = Case(
    When(targetTime__lt=now, then=now + timedelta(days=XX)),
    default=F('targetTime') + timedelta(days=XX)
)
object.save(update_fields=['targetTime'])

For debugging, try running this right after save to see what SQL queries have just run:

import pprint
from django.db import connection
pprint.pprint(["queries", connection.queries])

I've tested this with integers and it works in Django 1.8, I haven't tried dates yet so it might need some tweaking.