title: django admin populate fields in inlined models
body: <p>i am working on a website for a private school, in witch i have to calculate for each month a total price based on list of chosen services.</p>
  
  <p>i am using 3 models</p>
  
  <pre><code>class Eleve(models.Model):
      ...
  
  class Tarif(models.Model):
  
      eleve = models.OneToOneField(Eleve, on_delete=models.CASCADE)
      assurance = models.PositiveSmallIntegerField(default="1000")
      frais = models.PositiveSmallIntegerField(default="750")
      transport = models.PositiveSmallIntegerField(default="350")
      garde = models.PositiveSmallIntegerField(default="250")
      ...
  
  class Facture(models.Model):
      month_choices = (
          ('9', 'Septembre'),
          ('10', 'Octobre'),
          ...
      )
      eleve = models.ForeignKey(Eleve, on_delete=models.CASCADE)
      month = models.CharField(max_length=3, choices=month_choices)
      frais = models.BooleanField(default=True)
      transport = models.BooleanField()
      garde = models.BooleanField()
      total = models.PositiveSmallIntegerField()
      avance = models.PositiveSmallIntegerField()
      date = models.DateField(default=datetime.datetime.today)
  </code></pre>
  
  <p>Eleve == Student</p>
  
  <p>in admin.py, the models Tarif and Facture are display as TabularInline like so:</p>
  
  <pre><code>class TarifInline(admin.TabularInline):
      model = Tarif
      formset = RequiredInlineFormSet
      can_delete = False
  
  class FactureInline(admin.TabularInline):
      model = Facture
  </code></pre>
  
  <p>what i need is when i check transport or/and garde, the total field should be filled with the sum of current_eleve.tarif.transport and cuurent_eleve.tarif.garde</p>
  
  <p>the field total must be stored in database for some cases..</p>
  
  <p>any help would be appreciated</p>