5. Stock Management

Products in django-SHOP, by default do not keep track on their quantity in stock. This is because the product model shall be as lean as possible, and properties shall only be added, if they are required.

There are many shops which simply don’t need to keep track their quantity in stock. Either because the products they sell are arbitrarily reproducible, or because they share their stock with an offline store and hence the availability is managed through other means, typically an ERP.

However, for concrete products, keeping track of their limited number of pieces in stock, is mandatory. For this purpose django-SHOP offers two mutual exclusive product extensions. Without configured stock management, products tagged as “available” are considered as available until eternity and as unlimited in quantity.

5.1. Product with Quantity

A simple approach to keep track on the product’s quantity in stock, is to store this information side by side with the product model. This shall be done by adding a model field quantity, using an IntegerField, PositiveIntegerField, SmallIntegerField, PositiveSmallIntegerField, FloatField or a DecimalField. The field type shall be the same as that used by the quantity-field in shop.models.cart.CartItemModel.

In addition to the field storing the quantity in stock, add the mixin class shop.models.product.AvailableProductMixin to the product model declaration. Example:

from django.db import models
from shop.models.product import BaseProduct, BaseProductManager, AvailableProductMixin

class MyProduct(AvailableProductMixin, BaseProduct):
    # other product fields

    quantity = model.PositiveIntegerField(default=0)

    class Meta:
        app_label = app_settings.APP_LABEL

    objects = BaseProductManager()

The latter mixin class overrides the two methods get_availability() and deduct_from_stock() taking into account that now the number of products in stock is limited.

5.2. Product with Inventory

Sometimes it is not enough to just know the current number of items of a certain product. Consider the use case, where a product is short in supply but the next incoming delivery is already scheduled. In this situation a merchant might want to inform its customers, that the wanted product isn’t deliverable now but soon. Therefore, instead of adding a simple field storing the current quantity, we add a relation for each delivered charge. This model then holds a timestamp, when the next incoming delivery is expected. Example:

from django.db import models
from shop.models.product import BaseProduct, BaseProductManager
from shop.models.inventory import BaseInventory, AvailableProductMixin

class MyProduct(AvailableProductMixin, BaseProduct):
    # product fields

    class Meta:
        app_label = app_settings.APP_LABEL

    objects = BaseProductManager()

class MyInventory(BaseInventory):
    product = models.ForeignKey(

    quantity = models.PositiveIntegerField(default=0)

Since we have a relation from the inventory to our product model, we must use an InlineModelAdmin, while creating our admin backend. Example:

from django.contrib import admin
from myshop.models import MyProduct, Inventory

class MyInventoryAdmin(admin.StackedInline):
    model = MyInventory

class MyProductAdmin(admin.ModelAdmin)
    inlines = [MyInventoryAdmin]

This allows the merchant to schedule incoming deliveries.

5.2.1. Selling Short

If the timestamp for arrival is in between a short period of time, django-SHOP can sell short. Selling short means to sell something which you actually don’t own right now, but will own in the future. The period of time for selling short, can be configured using the settings directive SHOP_SELL_SHORT_PERIOD, using seconds or a Python timedelta object.

5.2.2. Time Limited Offer

An other possibility when using the Inventory relation, is to limit an offer for a determined period of time. This is when the merchant sets the field named latest to a time stamp in the near future. If this time stamp is in between the period configured using the settings directive SHOP_LIMITED_OFFER_PERIOD, then the customer is notified that this offer is limited in time.

5.3. Reserving Products in Cart

Products keeping track of their quantity in stock, either with the simple approach, or with the related inventory model have one behaviour in common – they deduct the number of items only during the purchase operation. For short term product types, this behaviour is impractical, because of the risk of overselling. Imagine a customer putting items into the cart and proceeding to checkout, only to discover that these items are not available in the moment he wants to purchase his items.

Instead we want to reserve items, whenever a customer puts them into his cart. Then these items are blocked for a certain period, normally only a few minutes, until they either have been puchased, or a timeout occured, making them available for other customers again.

Independently of the chosen approach, replace AvailableProductMixin with ReserveProductMixin in the product’s model declaration. Example for the simple approach:

from shop.models.product import BaseProduct, ReserveProductMixin

class MyProduct(ReserveProductMixin, BaseProduct):
    # product fields

Example using the related inventory model. Here we use the class ReserveProductMixin from the inventory module:

from shop.models.product import BaseProduct
from shop.models.inventory import ReserveProductMixin

class MyProduct(ReserveProductMixin, BaseProduct):
    # product fields

It is important to note, that when reserving products, it easily is possible to seemingly run short of products, because customers just add them to their cart, without actually buying them. This may result in a decrease of overall sales. Hence use this option only, if pending carts are flushed on a regular basis.

5.4. Prevent Overselling

in django-SHOP, purchasing the cart’s content is performed as one transaction. This means that either the cart is converted into an order as a whole, or left as it was before the customer clicked the Purchase Now-button.

Now consider the following race-condition: Two customers add the same product to their carts. The quantity of this product is limited in stock. As soon as one customer purchased this item, it is not available anymore for the other customer. In such a situation the whole purchasing operation is canceled for the second customer, so that he can look for an alternative product. If his purchasing operation is canceled, an informative message is displayed, saying that the product unexpectedly became unavailable.