Re: Prefetch object (django 1.7)

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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