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In reality, of course, an e-mail message contains other fields that the user might never see, but we'll limit our discussion to these four fields Each of these fields has different properties The To: and From: fields must contain valid e-mail addresses The Subject: field must contain a short line of text, and the body of the message can contain many paragraphs of text I've just described a simple data model for e-mail messages Notice I haven't mentioned data types or string lengths, nor have I defined "e-mail address" Data models are meant to be abstract representations Stay away from concrete definitions such as "The subject is stored as a Unicode string and can be up to 255 characters long" The reason we want to keep the data model abstract is that we're going to be defining these entities in multiple frameworks later Something that's represented by a string data type in our Java code, for instance, might be represented by a varchar data type in our database and as CDATA in our XML DTD We'll get into some of that later in s 5 and 6, but for now, let's keep things abstract At this point, you ask the second most important question (see the following section)
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What Do You Hope to Accomplish
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Asking what you hope to accomplish brings reality into the picture: What are the requirements of the system you're developing Let's look back at the requirements use cases:
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A user must be able to write an e-mail message A user must be able to send e-mail to another user A user must be able to read e-mail sent to him from another user An e-mail will consist of a subject line and a message body
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In examining our requirements, we've discovered that we actually have another element to contend with sers If our data model is to be of any use in application design, it must encompass users as well as e-mail messages themselves, and it must describe the relationship between users and e-mail messages This relationship is implicit in the preceding requirements: Users write and read e-mail messages
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Now we're back to our original questions: What is the data What data do we want to track about our users For simplicity's sake, we'll say that users consist of user names, full names, and passwords (because we don't want our users reading each other's mail, unless you're into that kind of thing)
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Making It Visual: Entity Relationship Diagrams
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Data modeling visualization techniques are many and varied, and it's not within the scope of this book to discuss all of them As discussed previously ee the sidebar in 3, Formal Use Case Analysis with Unified Modeling Language (UML) ML provides a useful set of tools for requirements capture and use case analysis It also provides a diagrammatic structure and an approach to data modeling The approach I've found most useful, and the one I've taken in this book, is to use a simplified version of UML In Figure 4-2, we see how we can begin to model our e-mail system and users, which can be referred to as user entities A box represents each entity The entity name is at the top of the box, and the attributes of that entity are listed below the entity name The line between the two boxes indicates that some kind of relationship exists between the two entities Again, notice that this data model is abstract, in that we're not defining data types or the nature of the relationship Figure 4-2 Data modeling diagram for e-mail system
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Roll Film: Back to CyberCinema
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In the previous chapter, we built a sample requirements document for a proposed CyberCinema movie review Web site In the case of CyberCinema, the answer to the question "What is the data " seems quite simple ovie reviews Application designers have to be suspicious by nature, and this answer is suspiciously simple Delving into this answer a little deeper, we can ask, "What are movie reviews " Movie reviews essentially are articles, and articles consist of paragraphs of written text The subject matter is movies Movie reviews are written by someone, so there's author information as well A data model begins to take shape, as shown in Figure 4-3
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