Running Signal when Related Field is Updated

I have a similar set up to this (simplified):

 

class Transaction(models.Model):
    sns = models.ManyToManyField(SN, related_name="transactions", blank=True)
    qty = models.FloatField()
    part = models.ForeignKey(Part, on_delete=models.CASCADE)


class InventoryTransaction(models.Model):   
    transaction = models.OneToOneField(Transaction, primary_key=True, on_delete=models.CASCADE)
    parent_sn = models.ForeignKey(SN, blank=True, null=True, on_delete=models.CASCADE)

 

 

class Calculations(models.Model):
    part = models.OneToOneField(Part, blank=False, null=False, on_delete=models.CASCADE, primary_key=True)
    i_qty = models.FloatField(blank=True, null=True)

 

 

Whenever the user creates a new inventory transaction or updates the qty in a transaction that has an inventory transaction, I want to be able to change Calculations.i_qty.  I’ve been using pre_save signals on the InventoryTransaction as the sender, but I’ve come to an obstacle.

 

When the user creates or deletes an inventory transaction, the calculation is perfect.  The problem comes when the user changes an inventory transaction because the sender should actually be Transaction, and not InventoryTransaction, since qty is on the Transaction field.  Sometimes I may update the Transaction instance without making any changes to the InventoryTransaction instance, so the signal is never called to update Calculations.i_qty.

 

However, if I modify the signal to change the sender to the Transaction model, I get problems when creating an InventoryTransaction because the relation hasn’t been established yet so we don’t know that it’s actually an InventoryTransaction that is being created.  If I make a signal to handle both Transaction and InventoryTransaction, the calculation is performed multiple times, which provides bad results.

 

I’ve considered using transaction atomicity.  Maybe I’m just missing something in my signals.  How do you typically handle this type of situation?

Thank you in advance!