Delete Django QuerySet objects with a duplicate value in a particular field

Question

I have this Django model (from Django CMS):

class Placeholder(models.Model):
    slot = models.CharField(_("slot"), max_length=50, db_index=True)
    default_width = models.PositiveSmallIntegerField(_("width"), null=True)

I want to delete the Placeholder objects with a duplicate 'slot' value, keeping only the first one of each and deleting the others.

How do I write a query (using the Django QuerySet API) that does this?

Rafen · Accepted Answer · 2011-12-01 20:45:15Z

5

You can try Torsten solution but using a dictionary instead, is way much faster.

existing_slots = {}
for placeholder in Placeholder.objects.all():
    if existing_slots.get(placeholder.slot, False):
        placeholder.delete()
    else:
        existing_slots[placeholder.slot] = True

answered Dec 1, 2011 at 20:45

Rafen

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1 Comment

unode Over a year ago

While you are at it, why not using a set() instead and drop the True. If it's efficiency you are after, a set() is more efficient from a memory point of view than a dictionary.

Torsten Engelbrecht · Accepted Answer · 2011-03-30 03:11:29Z

4

I would do a functional approach, rather than one particular query which does all of this:

existing_slots = []
for placeholder in Placeholder.objects.all():
    if placeholder.slot in existing_slots:
        placeholder.delete()
    else:
        existing_slots.append(placeholder.slot)

answered Mar 30, 2011 at 3:11

Torsten Engelbrecht

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