python - Django Delete all but last five of queryset

33
votes

I have a super simple django model here:

class Notification(models.Model):
    message = models.TextField()
    user = models.ForeignKey(User)
    timestamp = models.DateTimeField(default=datetime.datetime.now)

Using ajax, I check for new messages every minute. I only show the five most recent notifications to the user at any time. What I'm trying to avoid, is the following scenario.

User logs in and has no notifications. While the user's window is up, he receives 10 new messages. Since I'm only showing him five, no big deal. The problem happens when the user starts to delete his notifications. If he deletes the five that are displayed, the five older ones will be displayed on the next ajax call or refresh.

I'd like to have my model's save method delete everything but the 5 most recent objects whenever a new one is saved. Unfortunately, you can't use [5:] to do this. Help?

EDIT

I tried this which didn't work as expected (in the model's save method):

    notes = Notification.objects.filter(user=self.user)[:4]
    Notification.objects.exclude(pk__in=notes).delete()

i couldn't find a pattern in strange behavior, but after a while of testing, it would only delete the most recent one when a new one was created. i have NO idea why this would be. the ordering is taken care of in the model's Meta class (by timestamp descending). thanks for the help, but my way seems to be the only one that works consistently.

3
pythondjango
Useful to underline that the fact that you can't use delete on a slice is explained in the doc: docs.djangoproject.com/en/dev/ref/models/querysets/#delete - it raises a Cannot use 'limit' or 'offset' with delete error. I was hoping that since '09 something had changed but the "for" solution still seems to be the only one! – Stefano

3 Answers

48
votes

This is a bit old, but I believe you can do the following:

notes = Notification.objects.filter(user=self.user)[:4]
Notification.objects.exclude(pk__in=list(notes)).delete()  # list() forces a database hit.

It costs two hits, but avoids using the for loop with transactions middleware.

The reason for using list(notes) is that Django creates a single query without it and, in Mysql 5.1, this raises the error

(1235, "This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'")

By using list(notes), we force a query of notes, avoiding this. This can be further optimized to:

notes = Notification.objects.filter(user=self.user)[:4].values_list("id", flat=True)  # only retrieve ids.
Notification.objects.exclude(pk__in=list(notes)).delete()
12
votes

Use an inner query to get the set of items you want to keep and then filter on them.

objects_to_keep = Notification.objects.filter(user=user).order_by('-created_at')[:5]
Notification.objects.exclude(pk__in=objects_to_keep).delete()

Double check this before you use it. I have found that simpler inner queries do not always behave as expected. The strange behavior I have experienced has been limited to querysets that are just an order_by and a slice. Since you will have to filter on user, you should be fine.

9
votes

this is how i ended up doing this.

    notes = Notification.objects.filter(user=self.user)
    for note in notes[4:]:
        note.delete()

because i'm doing this in the save method, the only way the loop would ever have to run more than once would be if the user got multiple notifications at once. i'm not worried about that happening (while it may happen it's not likely to be enough to cause a problem).