Adding a new related entry using the "+" sign from M2M field doesn't update lists.

[devin13cox]

Related to Ticket #34789, PR ​https://github.com/django/django/pull/17897

Models:

from django.db import models


class State(models.Model):
    label = models.CharField(max_length=255)

    def __str__(self):
        return self.label


class Transition(models.Model):
    source = models.ManyToManyField(State, related_name="transition_source")
    target = models.ForeignKey(State, related_name="transition_target", on_delete=models.CASCADE)

Admin:

from django.contrib import admin

from .models import State, Transition


class TransitionAdmin(admin.ModelAdmin):
    filter_horizontal = ['source']


admin.site.register(State)
admin.site.register(Transition, TransitionAdmin)

Steps to Reproduce:

Add a State via the "+" on the M2M field to add to the Chosen Source column. We would expect the dropdown for the Target to contain the new value, but it does not.

Resolution as suggested by @nessita on the provided PR:

"For this, I tracked down the issue to the selector used in dismissAddRelatedObjectPopup to decide whether to call updateRelatedSelectsOptions or not. Basically the id used in the document.getElementById is wrong for the M2M widget (but correct for the single FK field, so we may need to fix the call site to pass the proper name)"

--- a/django/contrib/admin/static/admin/js/admin/RelatedObjectLookups.js
+++ b/django/contrib/admin/static/admin/js/admin/RelatedObjectLookups.js
@@ -119,7 +119,7 @@
 
     function dismissAddRelatedObjectPopup(win, newId, newRepr) {
         const name = removePopupIndex(win.name);
-        const elem = document.getElementById(name);
+        const elem = document.getElementById(name + '_from');
         if (elem) {
             const elemName = elem.nodeName.toUpperCase();
             if (elemName === 'SELECT') {

[devin13cox]

​https://github.com/django/django/pull/18026

[Natalia Bidart]

​Per PR comment