On May 22, 7:14 am, KentH <k...@khauser.net> wrote:
> I am trying to configure a m2m relationship to be ordered based on data in
> the through table. I assumed an 'order' attribute in the through-table's
> Meta class would be honored, but it's not. Bug #11850 (closed: wontfix)
> relates to this problem, but with no pointer as to a work-around.
>
> As custom ordering would seem a prime use case for a custom through class
> on m2m relations, I'm surprised how hard it is.
>
> In my case, I'm trying to order digital assets (eg sheet music) into a
> number of folios (collections). Music can appear in several collections.
> Order is manually specified for each collection. An attribute in a custom
> through table is a natural solution, but it doesn't work.
>
> Other examples which come to mind: Authors and books, or songs and
> composers. Since books are alphabetized by lead author, the order matters.
> And we wouldn't want Django to list "Lennon & McCartney" songs as "McCartney
> & Lennon"
>
> Anyway, how is the best way to crack this nut? After studying
> `db.models.related` I tried building a custom manager on the related class,
> and override the `get_query_set` method to look for the through attribute
> on the manager to update the ordered_by clause. No luck -- but with
> prefetch interaction, this seems pretty fragile in any case.
>
> Thanks for any help.
>
> Kent.
So, you would want to do collection.songs.all() and have that ordered
by the sequence defined in the m2m table? I agree - this seems to be
somewhat hard currently. You would need to do [t.song for t in
Through.objects.filter(collection=collection).order_by('seq').select_related('song')]
or something like that... It might be nice if the related manager of
collection would have a special lookup 'through', so you could to
collection.songs.all().select_related('through'), or
collection.songs.all().order_by('through__seq'). But it would be
somewhat messy to do that currently. Implementing better custom
lookups could make this easier to implement.
- Anssi
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