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Listing 8.3 Many-to-many relationship between ITEMS and CATEGORIES tables
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ITEMS ITEM_ID TITLE ... CATEGORIES CATEGORY_ID NAME ... NOT NULL, PRIMARY KEY NOT NULL
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ITEMS table primary key NUMBER VARCHAR2(255) CATEGORIES table
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NOT NULL, PRIMARY KEY NOT NULL
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primary key NUMBER VARCHAR2(255)
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ITEMS table CATEGORIES_ITEMS foreign key ITEM_ID NOT NULL, PRIMARY KEY NUMBER FOREIGN KEY(ITEMS(ITEM_ID)) CATEGORY_ID NOT NULL, PRIMARY KEY NUMBER FOREIGN KEY(CATEGORIES(CATEGORY_ID))
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Table linking CATEGORIES and ITEMS
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CATEGORIES_ ITEMS table foreign key
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The CATEGORIES_ITEMS table is called an association or intersection table and accomplishes a pretty neat trick. The only two columns it contains are foreign key references to the ITEMS and CATEGORIES tables (ironically the two foreign keys combined are the primary key for the table). In effect, it makes it possible to match up arbitrary rows of the two related tables, making it possible to implement many-tomany relationships. Since neither related table contains a foreign key, relationship direction is completely irrelevant. To get to the records on the other side of the relationship from either side, we must perform a join in the ORM layer involving the association layer. For example, to get all the items under a category, we must retrieve the CATEGORY_ID, join the CATEGORIES_ITEMS table with the ITEMS table, and retrieve all item data for rows that match the CATEGORY_ID foreign key. Saving the relationship into the database would involve saving a row CATEGORIES_ITEMS table that links rows stored in CATEGORIES and ITEM tables. Clearly, the many-tomany relationships are modeled differently in the relational world than they are in the OO world. Inheritance Unlike the three previous cases (one-to-one, one-to-many, and many-to-many), inheritance is probably the most severe case of the object-relational mismatch. Inheritance therefore calls for solutions that are not elegant fits to relational theory at all. The OO concept of inheritance has no direct equivalent in the relational world. However, there are a few creative ways that O/R solutions bridge this gap, including:
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The impedance mismatch
Storing each object in the inheritance hierarchy in completely separated tables Mapping all classes into a single table Storing superclass/subclasses in related tables
Because none of these strategies is simple, we ll save a detailed discussion for later in the chapter (section 8.4).
8.1.2 Introducing O/R mapping
In the most general sense, the term object-relational mapping means any process that can save an object (in our case a Java Object) into a relational database. As we mentioned, for all intents and purposes you could write home-brewed JDBC code to do that. In the realm of automated persistence, ORM means using primarily configuration metadata to tell an extremely high-level API which tables a set of Java Objects are going to be saved into. It involves the hopefully simple act of figuring out what table row an Object instance should be saved into and what field/ property data belongs in what column. In EJB 3, the configuration metadata obviously consists either of annotations or deployment descriptor elements, or both. As our impedance mismatch discussion points out, there are a few wrinkles in the idealistic view of automated persistence. Because of the inherent complexity of the problem, EJB 3 cannot make the solution absolutely effortless, but it goes a long way in making it less painful. In the next section, we start our discussion of EJB 3 ORM by covering the simple case of saving an entity without regard to domain relations or inheritance.
ORM portability in EJB 2
One of the greatest weaknesses of EJB 2 container-managed persistence (CMP) entity beans was that EJB 2 never standardized the process of ORM. Instead, mapping strategies were left up to the individual vendors, whose approaches varied widely. As a result, porting entity beans from one application server to another more or less meant redoing O/R mapping all over again. This meant that the portability that EJB 2 promised meant little more than empty words. EJB 3 firmly standardizes O/R mapping and gets us much closer to the goal of portability. As a matter of fact, as long as you are careful to steer clear of application server-specific features, you can likely achieve portability.
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