How to update the value from one model field in to value of another model field in Django?

Question

I want to update Contract_bills_amount field value in Class Contract from bills_mount field value in Class Bills:

#Contract

class Contract(models.Model):

    
    Contract_code = models.CharField(max_length=200)
    Contract_rec = models.DateTimeField('contract date ')
    Contract_total_amount = models.FloatField('total amount')
    Contract_deposit= models.FloatField('deposit')
    Contract_remainder_amount=models.FloatField('Contract remainder amount',null=True,blank=True)
    Contract_bills_amount=models.FloatField('bills_amount',null=True,blank=True)
    Contract_owner = models.ForeignKey(Client, on_delete=models.CASCADE)
    Contract_car = models.ForeignKey(Car, on_delete=models.CASCADE)


    def save(self, *args,**kwargs):
        
        a=self.Contract_total_amount
        b=self.Contract_deposit
        
        self.Contract_remainder_amount=a-b
        super(Contract,self).save(*args,**kwargs)


    def __str__(self):
            return self.Contract_code


#Bills

class bills(models.Model):

    bills_Contract = models.ForeignKey(Contract, on_delete=models.CASCADE)
    Contract_owner = models.ForeignKey(Client, on_delete=models.CASCADE)
    bills_code = models.CharField(max_length=200)
    Contract_rec = models.DateTimeField('date')
    bills_mount= models.FloatField('amount')




    def __str__(self):
            return self.bills_code

Answer 1

You already have a foreign key relationship between Contract and Bills as "bills_Contract". What else are you looking for? You only need to reference the value of the related table.

Example:

one_contract = Contract.objects.get(pk=1) #get the first contract
all_bills = one_contract.bills_set.all() #get all the bills in the first contract
desired_bill = all_bills.filter(bills_mount=400) #get bills amounting to 400

So, you may not need the "Contract_bills_amount" field in Contracts at all since you can already access that through backward relationship.

I see you have a reference to the Client in both Contract and Bills (perhaps for convenience but that cyclic reference is not necessary since the Contract Models identifies the owner.)

This relationship (i.e Bill --> Contract --> Client) implies that you also get the Bill from the client, like so: Bill <-- Contract <-- Client.

You need to learn more about referencing related objects in django