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Understanding object/relational persistence
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We re using the term full ORM to include any persistence layer where SQL is automatically generated from a metadata-based description. We aren t including persistence layers where the object/relational mapping problem is solved manually by developers hand-coding SQL with JDBC. With ORM, the application interacts with the ORM APIs and the domain model classes and is abstracted from the underlying SQL/JDBC. Depending on the features or the particular implementation, the ORM engine may also take on responsibility for issues such as optimistic locking and caching, relieving the application of these concerns entirely. Let s look at the various ways ORM can be implemented. Mark Fussel (Fussel, 1997), a developer in the field of ORM, defined the following four levels of ORM quality. We have slightly rewritten his descriptions and put them in the context of today s Java application development. Pure relational The whole application, including the user interface, is designed around the relational model and SQL-based relational operations. This approach, despite its deficiencies for large systems, can be an excellent solution for simple applications where a low level of code reuse is tolerable. Direct SQL can be fine-tuned in every aspect, but the drawbacks, such as lack of portability and maintainability, are significant, especially in the long run. Applications in this category often make heavy use of stored procedures, shifting some of the work out of the business layer and into the database. Light object mapping Entities are represented as classes that are mapped manually to the relational tables. Hand-coded SQL/JDBC is hidden from the business logic using wellknown design patterns. This approach is extremely widespread and is successful for applications with a small number of entities, or applications with generic, metadata-driven data models. Stored procedures may have a place in this kind of application. Medium object mapping The application is designed around an object model. SQL is generated at build time using a code-generation tool, or at runtime by framework code. Associations between objects are supported by the persistence mechanism, and queries may be specified using an object-oriented expression language. Objects are cached by the persistence layer. A great many ORM products and homegrown persistence layers support at least this level of functionality. It s well suited to medium-sized
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Object/relational mapping
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applications with some complex transactions, particularly when portability between different database products is important. These applications usually don t use stored procedures. Full object mapping Full object mapping supports sophisticated object modeling: composition, inheritance, polymorphism, and persistence by reachability. The persistence layer implements transparent persistence; persistent classes do not inherit from any special base class or have to implement a special interface. Efficient fetching strategies (lazy, eager, and prefetching) and caching strategies are implemented transparently to the application. This level of functionality can hardly be achieved by a homegrown persistence layer it s equivalent to years of development time. A number of commercial and open source Java ORM tools have achieved this level of quality. This level meets the definition of ORM we re using in this book. Let s look at the problems we expect to be solved by a tool that achieves full object mapping.
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Generic ORM problems
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The following list of issues, which we ll call the ORM problems, identifies the fundamental questions resolved by a full object/relational mapping tool in a Java environment. Particular ORM tools may provide extra functionality (for example, aggressive caching), but this is a reasonably exhaustive list of the conceptual issues and questions that are specific to object/relational mapping.
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What do persistent classes look like How transparent is the persistence tool Do we have to adopt a programming model and conventions for classes of the business domain How is mapping metadata defined Because the object/relational transformation is governed entirely by metadata, the format and definition of this metadata is important. Should an ORM tool provide a GUI interface to manipulate the metadata graphically Or are there better approaches to metadata definition How do object identity and equality relate to database (primary key) identity How do we map instances of particular classes to particular table rows How should we map class inheritance hierarchies There are several standard strategies. What about polymorphic associations, abstract classes, and interfaces
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