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8.4.4 Mapping polymorphic relationships
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In chapter 7 we explained that JPA fully supports inheritance and polymorphism. Now that we have completed our examination of mapping relationships and inheritance, you are probably anxious to learn about polymorphic associations. A relationship between two entities is said to be polymorphic when the actual relationship may refer to instances of a subclass of the associated entity. Assume that there is a bidirectional many-to-one relationship between the ContactInfo and
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Object-relational mapping
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User entities. We discussed earlier that User is an abstract entity and entities such as Bidder, Seller, and so forth inherit from the User class. When we retrieve the relationship field from the ContactInfo entity, the retrieved instance of the User entity will be an instance of its subclass, either Bidder or Seller. The greatest benefit of JPA is that you don t have to do any extra work to map polymorphic asso-
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ciations; you just define the relationship mapping between the superclass and the associated entity and the association becomes polymorphic.
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8.5 Summary
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In this chapter, we explored basic database concepts and introduced the objectrelational impedance problem. We reviewed ORM annotations such as @Table and @Column, and mapped some of the entities into database tables. We also reviewed the various types of primary key generation strategies as well as the mapping of composite primary keys. In addition, we examined the different types of relationships introduced in the previous chapter and showed you how to use JPA annotations such as @JoinColumn and @PrimaryKeyColumn to map those into database tables. You learned that manyto-many and unidirectional one-to-many relationships require association tables. We hope that the limitation to support unidirectional relationships using a target foreign key constraint will be addressed in a future release. We showed you the robust OO inheritance mapping features supported by JPA and compared their advantages and disadvantages. Of the three inheritance mapping strategies, using joined tables is the best from a design perspective. You should note, however, that we avoided some complexities in this chapter. First, we used field-based persistence in all of our code samples to keep them as short and simple as possible. Second, we only mentioned the most commonly used elements for the annotations featured in the chapter. We felt justified in doing so as most of the annotation elements we avoided are rarely used. We do encourage you to check out the full definition of all the annotations in this chapter available online at http://java.sun.com/products/persistence/javadoc-1_0-fr/javax/persistence/ package-tree.html. In the next chapter, you ll learn how to manipulate the entity and relationships we mapped in this chapter using the EntityManager API.
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Manipulating entities with EntityManager
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This chapter covers
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Persistence context and its scope Container- and application-managed entity managers Entity lifecycle and CRUD operations Detachment and merge operations Entity lifecycle listeners
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Manipulating entities with EntityManager
In chapter 7 you learned how to develop the application domain model using JPA. In chapter 8 you saw how domain objects and relationships are mapped to the database. While the ORM annotations we explored in chapter 8 indicate how an entity is persisted, the annotations don t do the actual persisting. This is performed by applications using the EntityManager interface, the topic of this chapter. To use an analogy, the domain model annotated with ORM configuration is like a children s toy that needs assembly. The domain model consists of the parts of the toy. The ORM configuration is the assembly instructions. While the assembly instructions tell you how the toy parts are put together, you do the actual assembly. The EntityManager is the toy assembler of the persistence world. It figures out how to persist the domain by looking at the ORM configuration. More specifically, the EntityManager performs CRUD (Create, Read, Update, Delete) operations on domain objects. The Read part includes extremely robust search and retrieval of domain objects from the database. This chapter covers each of the CRUD operations that the EntityManager provides, with the exception of the search part of search and retrieval. In addition to simple primary key based domain object retrieval, JPA provides SQL SELECT like search capabilities through the EJB 3 query API. The query API is so extensive and powerful that we ll dedicate chapter 10 to it while briefly touching on it in this one. Before we dive down into the persistence operations, you ll learn about the EntityManager interface, the lifecycle of entities, the concept of persistence context, and how to obtain an instance of EntityManager. We ll discuss entity lifecycle callback listeners before concluding with best practices. Before we get into too much code, we re going to gently introduce the EntityManager and briefly cover some concepts useful in understanding the nuances behind this critical part of JPA.
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