I suspect your usage of `distinct('path')` is causing issues during
prefetching here
as it will prevent different File with the same .path from being prefetched
for two different
folder objects.
You likely want to use `distinct('path', 'folder')` here instead where
'folder' might have a different
name based on whether or not you provided a related_name or
related_query_name to Folder.files.
Cheers,
Simon
Le lundi 30 décembre 2019 02:20:07 UTC-5, Mohit Solanki a écrit :
>
> Assume these two models.
>
> class Folder(model):
> name = models.CharField(max_length=8)
> files = models.ManyToManyField('File')
> class File:
> path = models.CharField(max_length=255)
>
>
> First I was fetching the folders and while looping over them fetching the
> files with a distinct path
>
> folders = Folder.objects.all()
> for folder in folders:
> files_with_distinct_path = folder.files.distinct('path').order_by('path')
>
>
> but this suffers from the N+1 problem, So I tried prefetching the files
> while fetching all the folders.
>
> from django.db.models import Prefetch
>
> folders = Folder.objects.prefetch_related(
> Prefetch(
> 'files', queryset=File.objects.distinct('path').order_by('path'),
> to_attr='distinct_files'
> ))
>
>
> but it turns out now there objects with empty `distinct_files` which is
> not the correct result, these objects have files but after prefetching, the
> result is empty only for few objects.
>
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