Polymorphic Basket Pattern

I have a design pattern I use when designing an e-Commerce system. I call it the polymorphic basket and as the name suggests, it is a design pattern covering the basket. However the basket is just a special case of an order (one that is stored, typically, in a session rather than in the database), and this pattern also covers orders stored in the database.

The problem the pattern seeks to address is maintaining and pricing a list of items. The naïve solution is to record a reference to the SKU, and a number representing the quantity. This solution does not generalise well. In many shops, there is a mixture of products conforming to different conceptual models. While some products can be fully represented by an SKU code, some need bespoke customisation. It is generally an easy task to create mixed catalogues and customisation pages for these products. Data storage for these products is perhaps simplest using a Concrete Table Inheritance pattern. Even if a given merchant is only selling within one model, they may one day want to supplement their product line with perhaps just a few products sold under a different model.

The pattern is to maintain a OrderItemList of polymorphic objects conforming to a OrderItem interface. On adding an item from the catalogue, the details are copied into an OrderItem of an appropriate type. The OrderItem must encapsulate a copy of catalogue data, not references (in case that data changes or is deleted). There are no situations I have come across where we need to query the database based on the contents of the OrderItemList, so persisting the OrderItems is an ideal case for using serialisation (the Serialized LOB pattern).

The nuances of the OrderItem interface come down to experience. The OrderItemList must also be able to correctly identify and handle duplication of OrderItems - if a compatible item is added, do they stack, remain as duplicates, or refuse to be added? If items are stackable, can shoppers change the quantity they wish to purchase? Can the maximum quantity purchasable vary? An OrderItem must be queried for a price, but how is this price affected by discounts and voucher codes? How does each item affect postage and packing options and costs?

In practice even this system is not sufficient because orders are not necessarily a flat list. In some cases, OrderItems must contain child OrderItems. These are things like add-on packs and upgrades which are conceptually self-contained, but can only be ordered alongside a parent item. Child items are priced seperately but grouped with the parent item for the purposes of removing the item from the basket or changing the quantity.

I include the following example list of items (derived from experience) which a flexible e-Commerce ordering system should be able to handle within a single basket:

• DVDs - for each product, one SKU and one price (many similar examples).
• Clothing - for each product, SKUs corresponding to both colour and size. Some sizes may have different prices (many similar examples).
• Groceries - for each product, SKUs corresponding to different pack sizes at different prices. (many similar examples).
• Computers - each SKU may be upgraded with a custom combination of add-ons, at extra cost. Some of these may be available as standalone products, other times not (likewise all configurable but mass-produced goods).
• Rope - pricing is based on the length of rope to be cut from the drum at a different rate per SKU. Users might choose length instead of quantity (likewise textiles).
• Kitchens/Worktops - each SKU corresponds to a finish, but pricing is complicated, based on how many boards need to be cut to satisfy a layout, given tolerances for carpentry and mitres, and the labour cost of performing that carpentry (likewise anything bespoke).
• Antiques - each antique can only be purchased by a single buyer and must then be removed unless/until the sale falls through (likewise anything second-hand).
• Samples - given away free, but limited to one of each SKU per customer. Because they are free the normal delivery charges may not apply, and the checkout might have to be cut short because payment information is not necessary. It may not even be worth combining these into the standard order process, although it might save the merchant some overhead if the customer simply wants a few samples to be chucked in when their real order is dispatched.