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Domain modeling and the JPA
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Figure 7.2 Entities are objects that can be persisted in the database. In the first step you identify entities for example, entities in the ActionBazaar domain.
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Putting in the links between objects that should know about each other (these are the infamously complex domain relationships) will complete our domain model. We encourage you to spend some time looking at figure 7.2 guessing how the objects might be related before peeking at the finished result in figure 7.3. We won t spell out every relationship in figure 7.3, since most are pretty intuitive even with the slightly cryptic arrows and numbers. We ll explain what is going on with the arrows and numbers in just a bit when we talk about direction and
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Figure 7.3 The ActionBazaar domain model complete with entities and relationships. Entities are related to one another and the relationship can be one-to-one, one-to-many, many-to-one, or manyto-many. Relationships can be either uni- or bidirectional.
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Implementing domain models
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multiplicity of relationships. For now, all you need to note is the text describing how objects are related to one another. For example, an item is sold by a seller, a seller may sell more than one item, the item is in one or more categories, each category may have a parent category, a bidder places a bid on an item, and so on. You should also note that although the domain model describes the possibilities for cobbling objects together, it does not actually describe the way in which the objects are manipulated. For instance, although you can see that an order consists of one or more items and is placed by a bidder, you are not told how or when these relationships are formed. But by applying a bit of common sense, it is easy to figure out that an item won through a winning bid is put into an order placed by the highest bidder. These relationships are probably formed by the business rules after the bidding is over and the winner checks out the item won. We ll clarify the concepts behind domain model objects, relationships, and multiplicity next before moving on to the JPA.
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7.1.3 Domain model actors Domain modeling theory identifies four domain model actors : objects, relationships, the multiplicity of relationships, and the optionality of relationships. Let s fill in the details that we have left out so far about all four actors.
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Objects From a Java developer perspective, domain objects are closely related to Java objects. Like Java objects, domain objects can have both behavior (methods in Java terms) and state (instance variables in Java). For example, the category domain object probably has the name, creation date, and modification date as attributes. Similarly, a category probably also has the behavior of being renamed and the modification date updated. There are likely hundreds of instances of category domain objects in the ActionBazaar, such as Junkyard Cars for Teenagers, Psychedelic Home D cor from the Sixties, Cheesy Romantic Novels for the Bored, and so on. Relationships In Java terms, a relationship is manifested as one object having a reference to another. If the Bidder and BillingInfo objects are related, there is probably a BillingInfo instance variable in Bidder, a Bidder instance variable in BillingInfo, or both. Where the object reference resides determines the direction of the arrows in figure 7.3. If Bidder has a reference to BillingInfo, the arrow should point from Bidder to BillingInfo. Or suppose Item and Bid have references to each other; an Item has Bids on it and Bids are placed on Items. Signifying this fact,
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Domain modeling and the JPA
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Rich vs. anemic domain models
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As we mentioned, domain models are eventually persisted into the database. It might already be obvious that it is easy to make the domain model objects look exactly like database tables. As a matter of fact, this is exactly why data modeling is often synonymous to domain modeling and DBAs (or data analysts) are often the domain experts. In this mode of thinking, domain objects contain attributes (mapping to database table columns) but no behavior. This type of model is referred to as the anemic model. A rich domain model, on the other hand, encapsulates both object attributes and behavior and utilizes objected-oriented design such as inheritance, polymorphism, and encapsulation. An anemic domain model may not necessarily be a bad thing for some applications. For one thing, it is painless to map objects to the database. As a rule of thumb, the richer the domain model is, the harder it is to map it to the database, particularly while using inheritance.
the arrow connecting Bid and Item points in both directions in figure 7.3. This is what is meant by a bidirectional relationship or association between Bid and Item as opposed to a unidirectional relationship or association between Seller and BillingInfo. Typically, objects are nouns and relationships are verbs such as has, is part of, is member of, belongs to, and so on. Multiplicity, or cardinality As you can probably infer from figure 7.3, not all relationships are one-to-one. That is, there may be more than one object on either side of a relationship. For example, a Category can have more than one Item. Multiplicity or cardinality refers to this multifaceted nature of relationships. The multiplicity of a relationship can be:
One-to-one Each side of the relationship may have at most only one object. An Employee object can have only one ID card and an ID card can only be assigned to one employee. One-to-many A particular object instance may be related to multiple instances of another. For example, an Item can have more than one Bid. Note that, taken from the point of view of a Bid, the relationship is said to be manyto-one. For example, many Bids can be placed by a Bidder in figure 7.3.
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