Error with inline on junction model: The inline value did not match the parent instance.

[ppo]

SOLVED while writing this so, let's keep it for reference…

TL;DR: As mentioned in the documentation:

my_order is the first field in the ordering tuple of the model’s Meta class.

The doc could be more explicit, for example:

my_order MUST ABSOLUTELY be the first field in the ordering tuple. It's retrieved first from the ModelAdmin, otherwise from the Model.Meta class.

FEATURE REQUEST: (already requested in #155, #198) Please allow to specify the order field in the Model.
It makes sense to sort first on another field in junction models. Especially as that ordering is used elsewhere.

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In inlines, the drag&drop works but there's just a global error message Please correct the errors below. but no specific errors in the form/formset… so it seems the error is on a missing or well hidden field.

Tracing down the problem, there's an error on a field in the inline's formset: The inline value did not match the parent instance.

Investigating in the HTML, apparently there's no order value and it's the album_id that is changed with the new sorting order (which explains the error message).

Investigating in the JS, it looks for a default_order_field.

Investigating in the admin.py, there's a _get_default_ordering function that finds it from the first item in the model_admin.ordering or model._meta.ordering… OK 🤬😕🙄

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"Conceptual" code…

class Media(Model): …

class Album(Model):
    medias = models.ManyToManyField(Media, through="AlbumMedias")

class AlbumMedias(Model):
    album, media, order
    class Meta:
        ordering = ["album", "order"]  # /!\ WRONG! `order` must be the first item.

class AlbumAdminInline(SortableAdminInlineMixin, TabularInline):
    model = Album.medias.through
    # Try 1: No fields defined
    # Try 2: fields = ["media"]
    # Try 3: fields = ["order", "media"]

class AlbumAdmin(SortableAdminMixin, ModelAdmin):
    inlines = [AlbumAdminInline]

    def save_form(self, request, form, change):
        obj = super().save_form(request, form, change)
        breakpoint()
        return obj

Debug:

> /…/my-app/admin.py(131)save_form()
-> return obj
(Pdb) n
--Return--
> /…/my-app/admin.py(131)save_form()-><Album: Foo>
-> return obj
(Pdb) n
> /…/django/contrib/admin/options.py(1578)_changeform_view()
-> formsets, inline_instances = self._create_formsets(request, new_object, change=not add)
(Pdb) n
> /…/django/contrib/admin/options.py(1579)_changeform_view()
-> if all_valid(formsets) and form_validated:
(Pdb) formsets[0].errors
[{'album': ['The inline value did not match the parent instance.']}, …(same for all inline rows, of course)…]

Admin page before drag&drop:

<td class="original">
  …
  <input type="hidden" name="album_medias-0-id" value="123" id="id_album_medias-0-id">
  <input type="hidden" name="album_medias-0-album" value="987" id="id_album_medias-0-album">
</td>
<td class="original">
  …
  <input type="hidden" name="album_medias-1-id" value="124" id="id_album_medias-1-id">
  <input type="hidden" name="album_medias-1-album" value="987" id="id_album_medias-1-album">
</td>

Admin page after drag&drop:

<td class="original">
  …
  <input type="hidden" name="album_medias-1-id" value="124" id="id_album_medias-1-id">
  <input type="hidden" name="album_medias-1-album" value="1" id="id_album_medias-1-album">
</td>
<td class="original">
  …
  <input type="hidden" name="album_medias-0-id" value="123" id="id_album_medias-0-id">
  <input type="hidden" name="album_medias-0-album" value="2" id="id_album_medias-0-album">
</td>