My space web page - CHAPTER 3 Domain models and metadata difference between
CHAPTER 3 Domain models and metadata difference between the two; they re merely different starting points. In the end, we re most interested in the structure and relationships of the business entities, the rules that have to be applied to guarantee the integrity of data (for example, the multiplicity of relationships), and the logic used to manipulate the data. In object modeling, there is a focus on polymorphic business logic. For our purpose and top-down development approach, it s helpful if we can implement our logical model in polymorphic Java; hence the first draft as an object-oriented model. We then derive the logical relational data model (usually without additional diagrams) and implement the actual physical database schema. Let s see the outcome of our analysis of the problem domain of the Caveat- Emptor application. 3.1.2 The CaveatEmptor domain model The CaveatEmptor site auctions many different kinds of items, from electronic equipment to airline tickets. Auctions proceed according to the English auction strategy: Users continue to place bids on an item until the bid period for that item expires, and the highest bidder wins. In any store, goods are categorized by type and grouped with similar goods into sections and onto shelves. The auction catalog requires some kind of hierarchy of item categories so that a buyer can browse these categories or arbitrarily search by category and item attributes. Lists of items appear in the category browser and search result screens. Selecting an item from a list takes the buyer to an item-detail view. An auction consists of a sequence of bids, and one is the winning bid. User details include name, login, address, email address, and billing information. A web of trust is an essential feature of an online auction site. The web of trust allows users to build a reputation for trustworthiness (or untrustworthiness). Buyers can create comments about sellers (and vice versa), and the comments are visible to all other users. A high-level overview of our domain model is shown in figure 3.2. Let s briefly discuss some interesting features of this model. Each item can be auctioned only once, so you don t need to make Item distinct from any auction entities. Instead, you have a single auction item entity named Item. Thus, Bid is associated directly with Item. Users can write Comments about other users only in the context of an auction; hence the association between Item and Comment. The Address information of a User is modeled as a separate class, even though the User may have only one Address; they may alternatively have three, for home, billing, and shipping. You do allow the user to have
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