Indeed, I am PostAuthor is a through model. Thanks I understand now how it
works. Prefetch('postauthors', PostAuthors.objects....) fails with
AttributeError: Cannot find 'postauthors' on Post object, 'postauthors' is
an invalid parameter to prefetch_related()
So it seems that 'postauthors' is not a valid prefetch lookup. I don't
need it, I just wanted to order Post.authors using the column
PostAuthor.created. Prefechting PostAuthors might need just using
PostAuthors.objects directly, or sth like this:
p = Prefetch('authors', User.objects.extra(select={'created':
'created'}).order_by('postauthor__created'))
Post.objects.prefetch_related(p).all()
I think it would be nice to add such examples to the docs. If that's the
case I can write a paragraph with an example like the one I am using, i.e.
with a custom through model.
Best regards,
Marcin Szamotulski
On 30 January 2014 05:23, Anssi Kääriäinen <[email protected]> wrote:
> To me it seems you are mixing the authors m2m relation and the through
> model. The 'authors' relates to User, not PostAuthor. But your queryset is
> pointing to PostAuthors.
>
> If you want to fetch PostAuthor instances, then you should likely prefetch
> with Prefetch('postauthors', PostAuthors.objects...). I am not exactly sure
> if the 'postauthor'
> lookup is right. It should be whatever post = models.ForeignKey(Post)'s
> reverse name is.
>
> - Anssi
>
>
> On Thursday, January 30, 2014 4:45:30 AM UTC+2, Marcin Szamotulski wrote:
>>
>> On 01:42 Mon 27 Jan , Marcin Szamotulski wrote:
>> > Hello,
>> >
>> > I have a model
>> >
>> > class Post(models.Model):
>> >
>> > ...
>> > authors = models.ManyToManyField('accounts.User',
>> through='PostAuthor', related_name='authors_posts')
>> >
>> >
>> > class PostAuthor(models.Model):
>> >
>> > user = models.ForeignKey('accounts.User')
>> > post = models.ForeignKey(Post)
>> > ...
>> >
>> >
>> > How can I use the Django 1.7 Prefetch object to load PostAuthors, this
>> > does not work:
>> >
>> > Post.objects.prefetch_related(
>> > Prefetch(
>> > 'authors',
>> > queryset=PostAuthors.object.select_related('user')
>> > )
>> >
>> > I got an exception:
>> > django.core.exceptions.FieldError: Cannot resolve keyword
>> 'authors_posts' into field. Choices are: created, id, post, post_id,
>> > user, user_id
>> >
>> > Thanks for help,
>> > Marcin Szamotulski
>>
>>
>> If somebody will search here is the solution I've found:
>>
>> Post.objects.prefetch_related(
>> Prefetch(
>> 'authors',
>> queryset=User.objects.order_by('postauthor__created'),
>> )
>>
>> It works because when prefetching User table is joined (inner joined)
>> with PostAuthor table and then the order_by('postauthor__created') will
>> make sense since PostAuthor has a columnt 'created'.
>>
>> It would be nice to have an example in the docs for that though.
>>
>> Best regards,
>> Marcin
>>
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