We recently had a client with a similar model, and I thought that this would be a good topic for a series of blog posts. For the purpose of this blog posts series, I am going to be using EBay as the example. Just to be clear, this is merely because it is a well known site that most people are likely to be familiar with.

In our model, we have the following Concepts:

• Categories
• Products
• Auctions
• Bids
• Sellers
• Buyers
• Orders

We will start from what is likely the most confusing aspect. Products and Auctions.

A Product is something that can be sold. It can be either a unique item “a mint condition Spiderman comics” or it can be something that we have lots of “a flying monkey doll”. There is a big distinction between the two, because of the way the business rules are structured.

Once an Auction has been started, it cannot be changed. This include everything about this auction, pricing, shipping information, product information, etc. But a Product may change at any time (maybe I now have the flying monkey in red and green, instead of the original red and yellow). That leads us to conclude that we actually have two instances of the Product in our domain. We have the Product Template (mutable, can change at any time) and with have the Auctioned Product (immutable).

That realization leads me to the following model for products and auctions:

{ "Name":"Flying Monkey Doll", "Colors":[ "Red & Yellow", "Blue & Green" ], "Price":29, "Weight":0.23 }	{ "Quantity":15, "Product":{ "Name":"Flying Monkey Doll", "Colors":[ "Blue & Green" ], "Price":29, "Weight":0.23 }, "StartsAt":"2011-09-01", "EndsAt":"2011-09-15" }
products/1	auctions/1

As you can see, the Auction is going to wholly own the product. Any change made to the product will not be reflected in the Auctioned Product. This has the advantage that we need only a single document load in order to show the auction page.

Another option would be to use the versioning bundle to do that, so we would have this:

{ "Name":"Flying Monkey Doll", "Colors":[ "Red & Yellow", "Blue & Green" ], "Price":29, "Weight":0.23 }	{ "Name":"Flying Monkey Doll", "Colors":[ "Blue & Green" ], "Price":29, "Weight":0.23 }	{ "Quantity":15, "Product": "products/1/versions/1", "StartsAt":"2011-09-01", "EndsAt":"2011-09-15" }
products/1	products/1/versions/1	auctions/1

The versioning bundle ensures that we can get immutable views of our documents, so we can safely reference the product by its id and version.

That is it for now, on the next post, we will deal with how to work with bids.