The following section describes the various components of Cartridge and the Django models used.


A category is the main approach for displaying products on the website. Categories in Cartridge are implemented as Mezzanine pages. Consult the Mezzanine documentation for a detailed overview of how pages are implemented in Mezzanine.


Products are the cornerstone of Cartridge and are made up from three separate models. First, the model Product provides the container for storing the core attributes of a product. The other two models are ProductImage and ProductVariation which each contain a ForeignKeyField to Product and can be accessed via Product.images and Product.variations respectively.

As with the Category model, the Product model inherits from the abstract model Displayable, which provides the model with features such as a URL/slug, and publish dates. It also inherits from the abstract model Priced which is discussed next. The fields provided by the Priced model are not editable via the admin. The rationale for this is discussed later in Denormalized Fields.

Priced Items

The Priced abstract model provides common features for an item that has a price. It contains fields such as Priced.unit_price for the base price and three fields that control whether the given item is on sale:

  • Priced.sale_price for the sale price
  • Priced.sale_from DateTimeField for the start of the sale
  • Priced.sale_to DateTimeField for the end of the sale

The Priced model also contains several convenience methods:

  • Priced.on_sale() for checking whether the current price is a sale price
  • Priced.has_price() for checking whether there is a current price at all
  • Priced.price() which returns the current price of either Priced.unit_price or Priced.sale_price if applicable

The Priced abstract model is inherited by the Product model previously discussed and the ProductVariation model discussed next.

Product Variations

The ProductVariation model represents a unique combination of options for a product. Consider a shirt that comes in two colours and three sizes: these combinations would be represented with a single Product instance for the shirt and six ProductVariation instances: one for each colour/size combination. These options are discussed below in Product Options.

The ProductVariation model is editable via the admin as the inline ProductVariationAdmin for the ProductAdmin admin; however, the attribute ProductVariationAdmin.extra is set to 0 and therefore ProductVariation instances cannot be added via the admin. Instead, when editing Product instances via the admin, a list of check-boxes is provided with the form, for each type of option available such as colour and size, with a check-box provided for each individual option. When the Product instance is saved, a set of related ProductVariation instances will be created for each combination of options that are checked. If no options are selected when creating a new Product instance, then a single ProductVariation instance will be created without any associated options. This means that a Product instances will always contains at least one related ProductVariation instance. All of this occurs via the ProductAdmin.save_formset() method and shows that the process of creating a new Product instance will always require two steps: first, creating the Product instance with its core attributes; and second, entering values for one or more related ProductVariation instances that are automatically created for each combination of options.

The ProductVariation model is dynamically constructed from the abstract model BaseProductVariation which defines all of the functionality discussed in this section, but for convenience is referred to as the ProductVariation model. The process of dynamically constructing the ProductVariation model exists in order to create an OptionField for each of the types of configured options such as colour or size. Like the Product model, the ProductVariation model inherits from the abstract model Priced. However, the fields provided by the Priced model in this case are editable in the admin via the inline admin ProductVariationAdmin.

The ProductVariation model also contains an SKUField ProductVariation.sku, which must be unique, and a ForeignKeyField ProductVariation.image which allows a ProductImage instance to optionally be related to the ProductVariation instance.

The ProductVariation model also contains a BooleanField ProductVariation.default which is used to specify which ProductVariation instance to use when displaying the related Product instance on the site. Only one ProductVariation instance can have this field set to True, and this constraint is managed within the ProductAdmin.save_formset() method referred to above.

Product Images

The ProductImage model is a simple container for storing an image file against a related Product instance. It contains an ImageField ProductImage.file and a CharField ProductImage.description which gives the image a meaningful description. The description provides a means of identifying the image so that it can be easily selected as the related image for the ProductVariation model which contains a nullable (optional) reference to the ProductImage model via the ForeignKeyField ProductVariation.image.

Denormalized Fields

Certain fields are duplicated for the Product model in order to avoid querying the database for ProductImage and ProductVariation instances when a large number of products are being iterated through on the site and the product’s image or price need to be displayed. These duplicate fields are provided by the Priced abstract model from which both the Product and ProductVariation models inherit, as well a CharField Product.image which stores the location of the image in the related ProductImage instance that is determined to be the default for display. The values for these fields are set for the Product instance when the ProductAdmin.save_formset() method is run as referred to above. The ProductVariation.default field is used to determine which ProductVariation instance’s Priced fields are duplicated. The ProductImage related to the ProductVariation instance is used for the Product.image field if selected; otherwise, the first ProductImage instance related to the Product instance is used.

Product Options

The ProductOption model provides a simple type and name for a selectable option for a ProductVariation instance (for example, Size: Small or Colour: Red). For performance and simplicity, these options don’t use a model relationship with the ProductVariation model but simply store the pool of available options. The configuration of available types such as colour and size is discussed in the section Configuration.


The Discount abstract model provides common features for the reduction of a price. It contains fields for three types of discounts:

  • Discount.discount_deduct for reducing by an amount
  • Discount.discount_percent for reducing by a percent
  • Discount.discount_exact for reducing to an amount

The Discount model also contains a DateTimeField Discount.valid_from and a DateTimeField Discount.valid_to, which together define the start and end dates of the discount, and a ManyToManyField Discount.categories and a ManyToManyField Discount.products, which together define the applicable Category and Product instances for which the discount is applicable.

The Discount abstract model is inherited by the DiscountCode and Sale models discussed next.

Discount Codes

The DiscountCode model provides a way for managing promotional codes that a customer can enter during the checkout process to receive a discount on an order. The DiscountCode model inherits from the Discount abstract model as referred to above and also contains fields such as DiscountCode.code for the promotional code to be entered, DiscountCode.min_purchase for specifying a minimum order total required for applying the discount, and a BooleanField DiscountCode.free_shipping which can be checked to provide free shipping for the discount code.


Discounts are applied to individual cart items when the discount code is assigned to one or more products (individually or by category) in the cart. If the discount code is not assigned to any products, the discount will be applied to the entire cart.


The Sale model provides a way for managing discounts across selections of Product instances. Like the DiscountCode model, the Sale model inherits from the abstract model Discount; however, the Sale model does not provide any extra fields. Instead it acts as a bulk update tool such that when a Sale instance is created or updated, it modifies the Product and related ProductVariation instances according to the selections made for Sale.categories and Sales.products. When this occurs, the various sale fields discussed in Priced Items such as Priced.sale_price, Priced.sale_from and Priced.sale_to are updated according to the type of discount given for either Sale.discount_deduct, Sale.discount_percent or Sale.discount_exact, and the dates given for Sale.valid_from and Sale.valid_to respectively. is also stored against Product and related ProductVariation instance such that if the Sale instance is updated or deleted the Product and related ProductVariation instances are updated with the relevant fields removed. This process occurs within the Sale._clear() method, which is called in both the and Sale.delete() methods.

This goal of this architecture is to decouple the sale information for each Product instance from the actual Sale instance so that no database querying is required in order to display sale information for a Product instance.


The Cart and related CartItem models represent a customer’s shopping cart. The Cart model provides the container for storing each CartItem instance. This model contains a customer manager CartManager which is assigned to Cart.objects. The CartManager contains the method CartManager.from_request() which, when given a request object, is responsible for creating a Cart instance and maintaining it across the session.

The Cart model contains the methods Cart.add_item() and Cart.remove_item() for modifying the cart, and also contains several convenience methods for use in templates that deal with the related CartItem instances, so as to avoid querying the database multiple times:

  • Cart.has_items() for checking if the Cart instance has related CartItem instances
  • Cart.total_quantity() for retrieving the total quantity of all the related CartItem instances
  • Cart.total_price() for retrieving the total price of all the related CartItem instances

The CartItem model represents each unique product in the customer’s Cart instance and inherits from the SelectedProduct abstract model discussed next.

Selected Products

The SelectedProduct abstract model represents a unique product and set of selected options that has been selected by a customer. The SelectedProduct model is inherited by the CartItem model previously discussed and the OrderItem model discussed next.

The SelectedProduct abstract model acts as a snapshot of a ProductVariation instance in that it does not contain a direct reference to the ProductVariation instance but rather copies information from it when the SelectedProduct instance is created. This is to ensure that any changes made to a ProductVariation instance do not affect existing SelectedProduct instances. The SelectedProduct model contains fields such as SelectedProduct.sku, SelectedProduct.unit_price and SelectedProduct.description, all of which are copied from the ProductVariation instance at creation time, with the SelectedProduct.description being created from the ProductVariation instances’s related Product.title as well as the selected options for the SelectedProduct instance. The SelectedProduct model also contains the IntegerField SelectedProduct.quantity for storing the selected quantity.

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