Update an instance of a model in django such that the older instance and its relationship with other instances remain unaffected

Question

I have been working on a e-commerce project. I have three models Item, OrderItem, Order. They are linked with Foreignkey(s) (Item -> OrderItem -> Order). Item is the actual product and an Order contain(s) Item(s).

Item basically represents a product. In Item there is an attribute 'price' which needs to updated as need suggest. Like during a sale or something else.

What happens is when I update the price of an Item, the price of that item also gets updated in the instances of the Order(s) that are already completed.

Basically I would want to separate these models in a way such that any changes in the Item model doesn't effect the Orders that are completed.

class Item(models.Model):
    title = models.CharField(max_length=100)
    sku = models.CharField(max_length=8, validators=[
                           MinLengthValidator(8)], unique=True)
    upc = models.CharField(max_length=12, validators=[
                           MinLengthValidator(12)], unique=True, blank=True, null=True)
    date_updated = models.DateTimeField(
        auto_now=True, blank=True, null=True)
    price = models.FloatField()
    discount_price = models.FloatField(blank=True, null=True)
    category = models.CharField(choices=CATEGORY_CHOICES, max_length=2)
    label = models.CharField(choices=LABEL_CHOICES, max_length=1)
    slug = models.SlugField()
    description = models.TextField()
    image = models.ImageField(upload_to=upload_location, blank=True, null=True)
    stock_quantity = models.IntegerField(default=0)

class OrderItem(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL,
                             on_delete=models.CASCADE)
    ordered = models.BooleanField(default=False)
    item = models.ForeignKey(Item, on_delete=models.CASCADE)
    item_variations = models.ManyToManyField(ItemVariation)
    quantity = models.IntegerField(default=1)
    purchase = models.FloatField(blank=True, null=True)


    def get_total_item_price(self):
        return self.quantity * self.item.price 

class Order(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL,
                             on_delete=models.CASCADE)
    ref_code = models.CharField(
        max_length=20, blank=True, null=True, unique=True)
    items = models.ManyToManyField(OrderItem)
    start_date = models.DateTimeField(auto_now_add=True)
    # Check this
    ordered_date = models.DateTimeField()
    # When the payment is made it becomes True
    ordered = models.BooleanField(default=False)
    shipping_address = models.ForeignKey(
        'Address', related_name='shipping_address', on_delete=models.SET_NULL, blank=True, null=True)
    billing_address = models.ForeignKey(
        'Address', related_name='billing_address', on_delete=models.SET_NULL, blank=True, null=True)
    payment = models.ForeignKey(
        'Payment', on_delete=models.SET_NULL, blank=True, null=True)
    coupon = models.ForeignKey(
        'Coupon', on_delete=models.SET_NULL, blank=True, null=True)
    being_delivered = models.BooleanField(default=False)
    received = models.BooleanField(default=False)
    refund_requested = models.BooleanField(default=False)
    refund_granted = models.BooleanField(default=False)
    refund_refused = models.BooleanField(default=False)

Any help will be appreciated, thank you.

Answer 1

You could have ItemPrice as a separate model with a One-to-Many relationship. Which prices for the item are stored with associated date changed.

models.py

class ItemPrice(models.Model):
    item = models.ForeignKey(Item, on_delete=models.CASCADE) 
    price = models.FloatField()
    date_changed = models.DateTimeField(auto_now=True, blank=True, null=True)

Then align your order date with the items price at that current time.