10. Cart and Checkout

In django-SHOP the cart’s content is always stored inside the database. In previous versions of the software, the cart’s content was kept inside the session for anonymous users and stored in the database for logged in users. Now the cart is always stored in the database. This approach simplifies the code and saves some random access memory, but adds another minor problem:

From a technical point of view, the checkout page is the same as the cart. They can both be on separate pages, or be merged on the same page. Since what we would normally name the “Checkout Page”, is only a collection of Cascade Plugins, we won’t go into further detail here.

10.1. Expired Carts

Sessions expire, but then the cart’s content of anonymous customers still remains in the database. We therefore must assure that these carts will expire too, since they are of no use for anybody, except, maybe for some data-mining experts.

By invoking

./manage.py shopcustomers
Customers in this shop: total=3408, anonymous=140, expired=88,
    active=1108, guests=2159, registered=1109, staff=5.

we gather some statistics about customers having visited of our django-SHOP site. In this example we see that 1109 customers bought as registered users, while 2159 bought as guests. There are 88 customers in the database, but they don’t have any associated session anymore, hence they can be considered as expired. Invoking

./manage.py shopcustomers --delete-expired

deletes those expired customers, and with them their expired carts. This task shall be performed by a cronjob on a daily or weekly basis.

10.2. Cart Models

The cart consists of two models classes Cart and CartItem, both inheriting from BaseCart and BaseCartItem respectively. As with most models in django-SHOP, these are using the Deferred Model Pattern, so that inheriting from a base class automatically sets the foreign keys to the appropriate model. This gives the programmer the flexibility to add as many fields to the cart, as the merchant requires for his special implementation.

In most use-cases, the default cart implementation will do the job. These default classes can be found at shop.models.defaults.cart.Cart and shop.models.defaults.cart_item.CartItem. To materialize the default implementation, it is enough to import these two files into the merchants shop project. Otherwise we create our own cart implementation inheriting from BaseCart and BaseCartItem. Since the item quantity can not always be represented by natural numbers, this field must be added to the CartItem implementation rather than its base class. Its field type must allow arithmetic operations, so only IntegerField, FloatField or DecimalField are allowed as quantity.


Assure that the model CartItem is imported (and materialized) before model Product and classes derived from it.

The Cart model uses its own manager. Since there is only one cart per customer, accessing the cart must be performed using the request object. We can always access the cart for the current customer by invoking:

from shop.models.cart import CartManager

cart = CartModel.objects.get_or_create_from_request(request)

Adding a product to the cart, must be performed by invoking:

from shop.models.cart import CartItemManager

cart_item = CartItemManager.get_or_create(
    cart=cart, product=product, quantity=quantity, **extras)

This returns a new cart item object, if the given product could not be found in the current cart. Otherwise it returns the existing cart item, increasing the quantity by the given value. For products with variations it’s not always trivial to determine if they shall be considered as existing cart items, or as new ones. Since django-SHOP can’t tell that difference for any kind of product, it delegates this question. Therefore the class implementing the shop’s products shall override their method is_in_cart. This method is used to tell the CartItemManager whether a product has already been added to the cart or is new.

Whenever the method cart.update(request) is invoked, the cart modifiers run against all items in the cart. This updates the line totals, the subtotal, extra costs and the final sum.

10.2.1. Watch List

Instead of implementing a separate watch-list (some would say wish-list), django-SHOP uses a simple trick. Whenever the quantity of a cart item is zero, this item is considered to be in the watch list. Otherwise it is considered to be in the cart. The train of though is as follows: A quantity of zero, never makes sense for items in the cart. On the other side, any quantity makes sense for items in the watch-list. Therefore reducing the quantity of a cart item to zero is the same as keeping an eye on it, without actually wanting it to purchase.

10.3. Cart Views

Displaying the cart in django-SHOP is as simple, as adding any other page to the CMS. Change into the Django admin backend and enter into the CMS page tree. At an appropriate location in that tree add a new page. As page title use “Cart”, “Basket”, “Warenkorb”, “Cesta”, or whatever is appropriate in the natural language used for that site. Multilingual CMS installations offer a page title for each language.

In the CMS page editor click onto the link named Advanced Settings at the bottom of the popup window. As template, choose the default one, provided it contains at least one big placeholder.

Enter “shop-cart” into the Id-field just below. This identifier is required by some templates which link directly onto the cart view page. If this field is not set, some links onto the cart page might not work properly.

It is suggested to check the checkbox named Soft root. This prevents that a menu item named “Cart” will appear side by side with other pages from the CMS. Instead, we prefer to render a special cart symbol located on the right of the navigation bar.

10.3.1. Cart using a Cascade Plugin

Click onto View on site and change into front-end editing mode to use the grid-system of djangocms-cascade. Locate the main placeholder and add a Row followed by at least one Column plugin; both can be found in section Bootstrap. Below that column plugin, add a child named Cart from section Shop. This Cart Plugin can be rendered in four different ways:

cart-structure Editable Cart

An “Editable Cart” is rendered using the Angular JS template engine. This means that a customer may change the number of items, delete them or move them to the watch-list. Each update is reflected immediately into the cart’s subtotal, extra fields and final totals.

Using the above structure, the rendered cart will look similar to this.


Depending on the chosen template, this layout may vary. Static Cart

An alternative to the editable cart is the static cart. Here the cart items are rendered by the Django template engine. Since here everything is static, the quantity can’t be changed anymore and the customer would have to proceed to the checkout without being able to change his mind. This probably only makes sense when purchasing a single product. Cart Summary

This only displays the cart’s subtotal, the extra cart fields, such as V.A.T., shipping costs and the final total. Watch List

A special view of the cart is the watch list. It can be used by customers to remember items they want to compare or buy sometimes later. The watch-list by default is editable, but does not allow to change the quantity. This is because the watch-list shares the same object model as the cart items. If the quantity of an item 0, then that cart item is considered to be watched. If instead the quantity is 1 ore more, the item is considered to be in the cart. It therefore is very easy to move items from the cart to the watch-list and vice versa. This concept also disallows to have an item in both the cart and the watch-list. This during online shopping, often can be a major point of confusion. Render templates

The path of the templates used to render the cart views is constructed using the following rules:

  • Look for a folder named according to the project’s name, ie. settings.SHOP_APP_LABEL in lower case. If no such folder can be found, then use the folder named shop.
  • Search for a subfolder named cart.
  • Search for a template named editable.html, static.html, watch.html or summary.html.

These templates are written to be easily extensible by the customized templates. To override the “editable cart” add a template with the path, say myshop/cart/editable.html to the projects template folder. This template then shall begin with {% extend "shop/cart/editable.html" %} and only override the {% block %}...{% endblock %} interested in.

Many of these template blocks are themselves embedded inside HTML elements such as <script id="shop/....html" type="text/ng-template">. The reason for this is that the editable cart is rendered in the browser by AngularJS using so called directives. Hence it becomes very straight-forward to override Angular’s script templates using Django’s internal template engine. Multiple templates

If for some special reasons we need different cart templates, then we must add this line to the projects settings.py:

    'ShopCartPlugin': (
        (None, _("default")),  # the default behavior
        ('myproject/cart/other-editable.html', _("extra editable")),

This will add an extra select button to the cart editor. The site administrator then can choose between the default template and an extra editable cart template. Proceed to Checkout

On the cart’s view, the merchant may decide whether to implement the checkout forms together with the cart, or to create a special checkout page onto which the customer can proceed. From a technical point of view, it doesn’t make any difference, if the cart and the checkout are combined on the same CMS page, or if they are split across two or more pages. In the latter case simply add a button at the end of each page, so that the customer can easily proceed to the next one.

On the checkout page, the customer has to fill out a few forms. These can be a contact form, shipping and billing addresses, payment and shipping methods, and many more. Which ones depend on the configuration, the legal regulations and the requirements of the shop’s implementation. In Cascade Plugins all shop specific CMS plugins are listed. They can be combined into whatever makes sense for a successful checkout.

10.3.2. Add a Cart via manually written Cart Template

Instead of using the CMS plugin system, the template for the cart can also be implemented manually. Based on an existing page template, locate the element, where the cart shall be inserted. Then use one of the existing templates in the folder django-shop/shop/templates/shop/cart/ as a starting point, and insert it at an appropriate location in the page template. Next, in the project’s settings.py, add this specialized template to the list CMS_TEMPLATES and select it for that page.

From a technical point of view, it does not make any difference whether we use the cart plugin or a handcrafted template. If the HTML code making up the cart has to be adopted to the merchants needs, we normally are better off and much more flexible, if we override the template code as described in section Render templates. Therefore, it is strongly discouraged to craft cart and checkout templates by hand.

10.4. Cart Modifiers

Cart Modifiers are simple plugins that allow the merchant to define rules in a programmatic way, how the totals of a cart are computed and how they are labeled. A typical job is to compute tax rates, adding discounts, shipping and payment costs, etc.

Instead of implementing each possible combination for all of these use cases, the django-SHOP framework offers an API, where third party applications can hooks into every computational step. One thing to note here is that Cart Modifiers are not only invoked, when the cart is complete and the customer wants to proceed to the checkout, but also for each item before being added to the cart.

This allows the programmer to vary the price of certain items, depending on the current state of the cart. It can for instance be used, to set one price for the first item, and other prices for every further items added to the cart.

Cart Modifiers are split up into three different categories: Generic, Payment and Shipping. In the shops settings.py they must be configured as a list or tuple such as:


Generic modifiers are applied always. The Shipping and Payment modifiers are applied only for the selected shipping and/or payment method. If the customer has not yet decided, how to ship or how to pay, then the corresponding modifiers are not applied.

When updating the cart, modifiers are applied in the order of the above list. Therefore it makes a difference, if taxes are applied before or after having applied the shipping costs.

Moreover, whenever in the detail view the quantity of a product is updated, then all configured modifiers are ran for that item. This allows the ItemModelSerializer, to even change the unit price of a product, depending on the total content of the cart.

Cart modifiers are easy to write and they normally consist only of a few lines of code. It is the intention of django-SHOP to seed an eco-system for these kinds of plugins. Besides computing the total, cart modifiers can also be used to sum up the weight, if the merchant’s product models specifies it.

Here is an incomplete list of some useful cart modifiers:

10.4.1. Generic Cart Modifiers

These kinds of cart modifiers are applied unconditionally onto the cart. A typical instance is the DefaultCartModifier, the CartIncludeTaxModifier or the CartExcludeTaxModifier. DefaultCartModifier

The shop.modifiers.default.DefaultCartModifier is required for almost every shopping cart. It handles the most basic calculations, ie. multiplying the items unit prices with the chosen quantity. Since this modifier sets the cart item’s line total, it must be listed as the first entry in SHOP_CART_MODIFIERS. Payment Cart Modifier

From these kinds of modifiers, only that for the chosen payment method is applied. Payment Modifiers are used to add extra costs or discounts depending on the chosen payment method. By overriding the method is_disabled a payment method can be disabled; useful to disable certain payments in case the carts total is below a certain threshold. Shipping Cart Modifier

From these kinds of modifiers, only that for the chosen shipping method is applied. Shipping Modifiers are used to add extra costs or discounts depending on chosen shipping method, the number of items in the cart and their weight. By overriding the method is_disabled a shipping method can be disabled; useful to disable certain payments in case the cart’s total is below a certain threshold or the weight is too high. How Modifiers work

Cart modifiers should extend the shop.modifiers.base.BaseCartModifier class and extend one or more of the given methods:


Until version 0.2 of django-SHOP, the Cart Modifiers returned the amount and label for the extra item rows, and django-SHOP added them up. Since Version 0.3 cart modifiers must change the line subtotals and cart total themselves.

class shop.modifiers.base.BaseCartModifier

Cart Modifiers are the cart’s counterpart to backends.

They allow to implement taxes, rebates, bulk prices, shipping- and payment costs in an elegant and reusable manner: Every time the cart is refreshed (via it’s update() method), the cart will call all subclasses of this modifier class registered with their full path in settings.SHOP_CART_MODIFIERS.

The methods defined here are called in the following sequence: 1. pre_process_cart: Totals are not computed, the cart is “rough”: only relations and quantities are available 1a. pre_process_cart_item: Line totals are not computed, the cart and its items are “rough”: only relations and quantities are available 2. process_cart_item: Called for each cart_item in the cart. The modifier may change the amount in cart_item.line_total. 2a. add_extra_cart_item_row: It optionally adds an object of type ExtraCartRow to the current cart item. This object adds additional information displayed on each cart items line. 3. process_cart: Called once for the whole cart. Here, all fields relative to cart items are filled. Here the carts subtotal is used to computer the carts total. 3a. add_extra_cart_row: It optionally adds an object of type ExtraCartRow to the current cart. This object adds additional information displayed in the carts footer section. 4. post_process_cart: all totals are up-to-date, the cart is ready to be displayed. Any change you make here must be consistent!

Each method accepts the HTTP request object. It shall be used to let implementations determine their prices, availability, taxes, discounts, etc. according to the identified customer, the originating country, and other request information.

arrange_watch_items(watch_items, request)

Arrange all items, which are being watched. Override this method to resort and regroup the returned items.

arrange_cart_items(cart_items, request)

Arrange all items, which are intended for the shopping cart. Override this method to resort and regroup the returned items.

pre_process_cart(cart, request, raise_exception=False)

This method will be called before the Cart starts being processed. It shall be used to populate the cart with initial values, but not to compute the cart’s totals.

  • cart – The cart object.
  • request – The request object.
  • raise_exception – If True, raise an exception if cart can not be fulfilled.
pre_process_cart_item(cart, item, request, raise_exception=False)

This method will be called for each item before the Cart starts being processed. It shall be used to populate the cart item with initial values, but not to compute the item’s linetotal.

  • cart – The cart object.
  • item – The cart item object.
  • request – The request object.
  • raise_exception – If True, raise an exception if cart can not be fulfilled.
process_cart_item(cart_item, request)

This will be called for every line item in the Cart: Line items typically contain: product, unit_price, quantity and a dictionary with extra row information.

If configured, the starting line total for every line (unit price * quantity) is computed by the DefaultCartModifier, which typically is listed as the first modifier. Posterior modifiers can optionally change the cart items line total.

After processing all cart items with all modifiers, these line totals are summed up to form the carts subtotal, which is used by method process_cart.

post_process_cart_item(cart, item, request)

This will be called for every line item in the Cart, while finally processing the Cart. It may be used to collect the computed line totals for each modifier.

process_cart(cart, request)

This will be called once per Cart, after every line item was treated by method process_cart_item.

The subtotal for the cart is already known, but the total is still unknown. Like for the line items, the total is expected to be calculated by the first cart modifier, which typically is the DefaultCartModifier. Posterior modifiers can optionally change the total and add additional information to the cart using an object of type ExtraCartRow.

post_process_cart(cart, request)

This method will be called after the cart was processed in reverse order of the registered cart modifiers. The Cart object is “final” and all the fields are computed. Remember that anything changed at this point should be consistent: If updating the price you should also update all relevant totals (for example).

add_extra_cart_item_row(cart_item, request)

Optionally add an ExtraCartRow object to the current cart item.

This allows to add an additional row description to a cart item line. This method optionally utilizes and/or modifies the amount in cart_item.line_total.

add_extra_cart_row(cart, request)

Optionally add an ExtraCartRow object to the current cart.

This allows to add an additional row description to the cart. This method optionally utilizes cart.subtotal and/or modifies the amount in cart.total.