class Outlet(models.Model):
    a = models.CharField(max_length=253, primary_key=True, db_index=True)
    def _split(self, idx):
        if not self.a:
            return None
        return self.a.split(".")[idx]
    @property
    def b(self):
        return self._split(0)
    @property
    def c(self):
        return self._split(1)

@admin.register(Outlet)
class OutletAdmin(admin.ModelAdmin):
    fields = ("a", "b", "c")
    readonly_fields = ("b", "c")