title: Update model field when creating from another model instance which is ForeignKey related
body: <p>I started car fleet app and created a few models in models related file, I did models hierarchy that on top I have the car brand, below ForeignKey related car models to each brand, and ForeignKey for each car id that eventually consists the car inventory,  which is called "class CarPool", in this model I have value "car_sale_status = models.BooleanField(default=False)", what I would like to achieve is each time I create an instance from associated ForeignKey of the seller will update automatically "car_sale_status = models.BooleanField(default=False)" to True</p>
  
  <p>Please advice</p>
  
  <p>Thanks</p>
  
  <pre><code>class CarPool(models.Model):
  
      # Relationship Fields
      car_inventory = models.ForeignKey(
          'cardealer.CarTypes',
          on_delete=models.CASCADE, related_name="carpools", 
      )
      car_id = models.CharField(max_length=30,default="")
      car_location = GeopositionField()
      car_sale_status = models.BooleanField(default=False)
  
  
      YEAR_CHOICES = []
  
  
      for r in range(1980, (datetime.datetime.now().year+1)):
          YEAR_CHOICES.append((r,r))
  
  
      year_manufacture = models.IntegerField(('year_manufacture'), choices=YEAR_CHOICES, default=datetime.datetime.now().year)
  
  
      def __str__(self):
          return self.car_id
  
      def __unicode__(self):
          return self.car_inventory
  
  class CustomerBuyer(models.Model):
  
      buyer_car_buy = models.ForeignKey(
          'cardealer.CarPool',
          on_delete=models.CASCADE, related_name="customer_buyer",default='0'
      )
      buyer_private_name = models.CharField(max_length=80,default='')
      buyer_family_name = models.CharField(max_length=80,default='')
      buyer_private_id = models.IntegerField(default=0)
      transaction_date = models.DateTimeField(default=datetime.datetime.now())
      transaction_amount = models.IntegerField(default=0)
  
  
  
  
      def __str__(self):
          return self.buyer_private_name + ' '+ self.buyer_family_name
  
      def __unicode__(self):
          return self.buyer_private_name + ' '+ self.buyer_family_name
  </code></pre>