Django annotate queryset depending on another query

Question

class TradeItem(models.Model):
    ...

class Wishlist(models.Model):
    user = models.ForeignKey(User, related_name='wishlist_user')
    item = models.ForeignKey(TradeItem, related_name='wishlist_item')

I need to make TradeItem queryset annotated with a Boolean of whether an item is in this user's wishlist. Something like

items = TradeItem.objects.all().annotate(wishlist=
 <heresy>
 if Wishlist.objects.filter(user=request.user, item={current_item}: 
  True 
 else: 
  False
 </heresy>
 )

Is it possible to do the said thing with annotate/aggregate, and what would be the most efficient way for that in general?

webtweakers · Accepted Answer · 2020-06-03 17:57:50Z

4

You can actually solve this on database level in a single query, using Exists:

from django.db.models import Exists, OuterRef
user_wishlist = Wishlist.objects.filter(item_id=OuterRef('id'), user=user)
items = TradeItem.objects.annotate(in_wishlist=Exists(user_wishlist))

PS: I know, old question...

answered Jun 3, 2020 at 17:57

webtweakers

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Nerade · Accepted Answer · 2017-02-20 12:41:52Z

3

The problem with this is that annotations are happening on database (SQL) level. You can only make use of functions which are nativly supported by the SQL dialect (like SUM, AVG...). If you want to append an arbitrary attribute/data field you should iterate over the queryset and check this for every instance seperatly:

items = TradeItem.objects.all()
user_wishlist = Wishlist.objects.filter(user=request.user)
for item in items:
    item.wishlist = user_wishlist.filter(item=item).exists()

answered Feb 20, 2017 at 12:41

Nerade

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2 Comments

Vémundr Over a year ago

Thank you, I started using your method, and it made my code neater. I sadly did not know that you can do qs.something = something , so terrible.

devordem Over a year ago

Iterating over a queryset might come with performance problems

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