class A(models.Model):
    name = models.CharField(max_length=100)
    c = models.ForeignKey(C, related_name='letters_c', null=True, blank=True)

    def __unicode__(self):
        return self.name


class B(models.Model):
    id= models.CharField(max_length=200, blank=False, default="")
    a = models.ForeignKey(A, related_name='letters_a', default=0)

    def __unicode__(self):
        return self.identifier

class A(admin.ModelAdmin):
fields = ['name', 'c', 'letters_a']
list_display = ['name']
ordering = ['name']