Django prefetch_related with limit

Question

Is there a way to tell prefetch_related to only fetch a limited set of related objects? Lets say I am fetching a list of users and I know I want to fetch their

Answer 1

The only way to limit the number of prefetched related objects seems to be using Prefetch() and filtering on fileds. Using sliceing

User.objects.all().prefetch_related(
    Prefetch('msg_sent', queryset=UserMsg.objects.order_by('-created')[:10]))

returns an error

AssertionError: Cannot filter a query once a slice has been taken.

The only way to limit the number of related objects seems to be using filter on a value, for example

from datetime import datetime, timedelta
timelimit = datetime.now() - timedelta(days=365)

User.objects.all().prefetch_related(
    Prefetch('msg_sent', queryset=UserMsg.objects.filter(created__gte=timelimit)))

While that doesn't return a fixed number, in may be useful in some situation, and it will reduce the number of prefetched objects.

Answer 2

I think there is a workaround now to in django new version as we have OuterRef and Subquery.

from django.db.models import OuterRef, Subquery, Prefetch

subqry = Subquery(Comment.objects \
    .filter(user_id=OuterRef('user_id')) \
    .values_list('id', flat=True)[:5])

User.objects.prefetch_related(
    Prefetch('comments', queryset=Comment.objects.filter(id__in=subqry)))

Answer 3

thats what actually works for me django(2.1) (based on haseebahmad answer).
in order for prefetch_related to accept customize queryset: Prefetch
so:
from django.db.models import OuterRef, Subquery ,Prefetch

User.objects.all().prefetch_related(Prefetch('comment_set',  
queryset=Comment.objects.filter(id__in= 
Subquery(Comment.objects.filter(user_id=OuterRef('user_id')).
values_list('id', flat=True)[:1]))))