Save dan auto update beda class di django admin

from __future__ import unicode_literals

from django.db import models
from karyawan.models import Karyawan
from karyawan.models import Divisi
from django.conf import settings
from django.http import HttpResponse
#Create your models here.
class Mutasi(models.Model):
    JENIS_MUTASI_CHOICES = (
        ('MASUK', 'MASUK'),
        ('PINDAH DIVISI', 'PINDAH DIVISI'),
        ('PINDAH LAIN DIVISI', 'PINDAH LAIN DIVISI'),
    )

    karyawan = models.ForeignKey(Karyawan)
    jenis_mutasi = models.CharField(max_length=20, choices=JENIS_MUTASI_CHOICES)
    divisi_awal = models.ForeignKey(Divisi, related_name='divisi_awal')
    divisi_tujuan = models.ForeignKey(Divisi, related_name='divisi_tujuan')
    tanggal_mutasi = models.DateField(default='DD-MM-YYYY')
    no_sk = models.CharField(max_length=25)

    def __unicode__(self):
        return self.karyawan.nama
        return self.divisi_awal.nama
        return self.divisi_tujuan

bagaimana cara agar saat kita simpan nanti bisa terupdate di class karyawan field divisi,

terima kasih..