Regression in allowing to handle M2M fields from the "receiving end" - follow up to #23329

[Emmanuelle Delescolle]

Up until Django 1.5.8 and Django 1.6.5 it was possible to have a ManyToMany field displayed in the admin on the "receiving-end" of the relationship with the ability to add new records.
Due to the restrictiveness of the new to_field_allowed method, one cannot open the popup for adding new records to the Model where the ManyToMany relationship is defined.

I realize this usage of the admin site is undocumented but, since the current code allows to perform that from the "receiving-end" of ForeignKey fields, IMHO it should also work for ManyToMany fields.

If the descriptions sounds too vague, see ​http://www.lasolution.be/blog/related-manytomanyfield-django-admin-site.html for a complete step-by-step explanation on how to reproduce.

[Emmanuelle Delescolle]

Failing test (patch for master)

[Emmanuelle Delescolle]

Fix

[Emmanuelle Delescolle]

If this is approved I'll create pull requests for 1.6, 1.7 and master

[Simon Charette]

Thanks for the detailed report. The issue seems valid and the patch seems looks like the correct approach.

Could you open a PR against master only with the following changes:

1. if opts.many_to_many and field.primary_key should do.
2. Since this check is cheap and doesn't require access to the set of registered model I would move it before they're collected.
3. Could you define an unreferenced model instead of relying on the Pizza one to make sure the additional checks are still covered if another model referring to Pizza is added in the future.
4. Could you add a comment with a reference to the ticket over the assertion.
5. Since this is a regression and will be backported you'll need to add the changes to the 1.4.16, 1.5.11, 1.6.8 and 1.7.1 release notes.

Let me know if you need additional help to land this.

[Emmanuelle Delescolle]

Nope, I think I git it: ​https://github.com/django/django/pull/3311

And thanks for the quick reply.

[Tim Graham <timograham@…>]

In a24cf217220dca44b7bd5b36ad9c14a96bca486e:

Fixed #23604 -- Allowed related m2m fields to be references in the admin.

Thanks Simon Charette for review.

[Tim Graham <timograham@…>]

In f58392d8d8b68e2e7777768f39941cd995ce16e4:

[1.4.x] Fixed #23604 -- Allowed related m2m fields to be references in the admin.

Thanks Simon Charette for review.

Backport of a24cf21722 from master

[Tim Graham <timograham@…>]

In c5c4bfa12aa0e8d4e8b46d77b6159f59330c3313:

[1.6.x] Fixed #23604 -- Allowed related m2m fields to be references in the admin.

Thanks Simon Charette for review.

Backport of a24cf21722 from master

[Tim Graham <timograham@…>]

In 314e9cd38f6bf72a619e5d07c99371439c33ba11:

[1.5.x] Fixed #23604 -- Allowed related m2m fields to be references in the admin.

Thanks Simon Charette for review.

Backport of a24cf21722 from master

[Tim Graham <timograham@…>]

In f8d845910b17c119b7ed7fe8b448800bd39c17bc:

[1.7.x] Fixed #23604 -- Allowed related m2m fields to be references in the admin.

Thanks Simon Charette for review.

Backport of a24cf21722 from master

[Erik Romijn <erik@…>]

In 29563cfb:

Added Emmanuelle Delescolle to AUTHORS.

(Fixed #23604 in 2014 and was not added before.)