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 occus 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)