'ManyToOneRel' object has no attribute 'get_limit_choices_to'

[Seb G]

Summary

The newly introduced feature in Django 3.2

ModelAdmin.autocomplete_fields now respects ForeignKey.to_field and ForeignKey.limit_choices_to when searching a related model)

Triggers a bug when used on ManyToOneRel:

Traceback (most recent call last):
  File "/home/me/.virtualenvs/myproject/lib/python3.7/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/home/me/.virtualenvs/myproject/lib/python3.7/site-packages/django/core/handlers/base.py", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/me/.virtualenvs/myproject/lib/python3.7/site-packages/django/contrib/admin/sites.py", line 250, in wrapper
    return self.admin_view(view, cacheable)(*args, **kwargs)
  File "/home/me/.virtualenvs/myproject/lib/python3.7/site-packages/django/utils/decorators.py", line 130, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/home/me/.virtualenvs/myproject/lib/python3.7/site-packages/django/views/decorators/cache.py", line 44, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "/home/me/.virtualenvs/myproject/lib/python3.7/site-packages/django/contrib/admin/sites.py", line 232, in inner
    return view(request, *args, **kwargs)
  File "/home/me/.virtualenvs/myproject/lib/python3.7/site-packages/django/contrib/admin/sites.py", line 417, in autocomplete_view
    return AutocompleteJsonView.as_view(admin_site=self)(request)
  File "/home/me/.virtualenvs/myproject/lib/python3.7/site-packages/django/views/generic/base.py", line 70, in view
    return self.dispatch(request, *args, **kwargs)
  File "/home/me/.virtualenvs/myproject/lib/python3.7/site-packages/django/views/generic/base.py", line 98, in dispatch
    return handler(request, *args, **kwargs)
  File "/home/me/.virtualenvs/myproject/lib/python3.7/site-packages/django/contrib/admin/views/autocomplete.py", line 25, in get
    self.object_list = self.get_queryset()
  File "/home/me/.virtualenvs/myproject/lib/python3.7/site-packages/django/contrib/admin/views/autocomplete.py", line 42, in get_queryset
    qs = qs.complex_filter(self.source_field.get_limit_choices_to())
AttributeError: 'ManyToOneRel' object has no attribute 'get_limit_choices_to'

Steps to reproduce

Use the following model:

class Ticket(models.Model)
    user = models.ForeignKey("users.User, on_delete=models.PROTECT, verbose_name=_("user"))

And the following admin:

@admin.register(Ticket)
class TicketAdmin(admin.ModelAdmin):
    autocomplete_fields = ("user",)