Django does not respect to_field's model on an inline model admin

[sebastien@…]

The problem occurs in the function __unicode__ of ModelB

When editing a ModelA instance in admin site, all ModelB instances have a modela_id set to the pk of the ModelA instance, instead of the value of the to_field's field. And, when accessing to the field modela of a ModelB instance, a DoesNotExist exception is raised.

This problem does not occur in the shell :

In [3]: ModelA.objects.all()[0].modelb_set.all()[0].modela_id
Out[3]: u'TRUC'

In [6]: ModelB.objects.all()[0].modela_id
Out[6]: u'TRUC'

See below to reproduce

# models.py

class ModelA(models.Model):
    code = models.CharField(max_length=20, unique=True)

class ModelB(models.Model):
    modela = models.ForeignKey(ModelA, to_field="code")
    position = models.IntegerField()

    def __unicode__(self):
        return u"%s" % self.modela

# admin.py

class ModelBInlineAdmin(admin.TabularInline):
    model = ModelB

class ModelAAdmin(admin.ModelAdmin):
    model = ModelA
    inlines = [ModelBInlineAdmin]

admin.site.register(ModelA, ModelAAdmin)

[gautier]

Same cause as #10243, #11043, fixed by [10756].

But the merging of soc2009/model-validation in [12098] broke the fix, by forcing the value of the related primary key into the foreign key field.

My patch removes this behaviour, and add a regression test. All other unit tests still pass.

[gautier]

Fix + regression test

[patchhammer]

fix_inline_model_with_to_field.diff fails to apply cleanly on to trunk

[gautier]

Fix + regression test, updated for new trunk

[Aymeric Augustin]

Change UI/UX from NULL to False.

[anonymous]

I updated the patch so that is does not change behavior at all for no to_field formsets. Any feedback or suggestions welcome.

This bug has been present for a while and would be nice to get fixed.

[Aymeric Augustin]

Added a bit of markup for reaadability.