python - Django - Add the value of new entry to an existing value when another object of same name is added to the table

Question

I'm fairly new to Django, hence open for any kind of suggestions

I have two models Order and Stock

My code:

models.py

class Order(models.Model):
    idorder = models.AutoField(db_column='idorder', primary_key=True)
    date_ordered = models.DateField(db_column='Date_Ordered', blank=True, null=True)  # Field name made lowercase.
    number_ordered = models.IntegerField(db_column='Number_Ordered', blank=True, null=True)  # Field name made lowercase.
    idproduct = models.ForeignKey(Product, models.DO_NOTHING, blank=True, db_column='idProduct', verbose_name='Product Name')  # Field name made lowercase.
    idsupplier = models.ForeignKey(Supplier, models.DO_NOTHING, blank=True, db_column='idSupplier', verbose_name='Supplier Name')  # Field name made lowercase.
    
    class Meta:
        managed = False
        db_table = 'order'

class Stock(models.Model):
    idstock = models.AutoField(primary_key=True)
    quantity = models.IntegerField(db_column='Quantity', blank=True, null=True)  # Field name made lowercase.
    idproduct = models.ForeignKey(Product, models.DO_NOTHING, db_column='idProduct', verbose_name='Product Name', blank=True)  # Field name made lowercase.
    idorder = models.ForeignKey(Order, models.DO_NOTHING, db_column='idOrder', verbose_name='Order Name', blank=True)

    class Meta:
        managed = False
        db_table = 'stock'

    def __str__(self):                  #to return supplier_name for foreign key references
        return self.idproduct.product_name

I have implemented the addition of corresponding Order object into Stock table using signals

#this signal is to add object to stock automatically when order object is created
def create_stock_item(sender, instance, created, **kwargs):
    if created:
        s = Stock(
            idorder=instance, 
            quantity=instance.number_ordered,
            idproduct=instance.idproduct
            )
        s.save()
post_save.connect(create_stock_item, sender=Order)

When I add an Order, the corresponding Stock object is created

What I want to achieve is that when I add another entry for Teddy Bear in the Order table, I want the quantity field for Teddy Bear in the Stock table to be the sum of old entry and new entry instead of creating a new entry.

Answer 1

等待大神答复