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Object-relational mapping concepts Mapping entities to tables Relationship mapping Inheritance mapping strategies

23. Erase the throw new. . . code. 24. Now we need to make sure that there was no error in the downloading process. To do that, we

In the previous chapter, we used EJB 3 JPA features to create a POJO domain model that supported a full range of OO features, including inheritance. We discussed entities, embedded objects, and the relationships between them using EJB 3 annotations. In this chapter, you ll learn how to persist our domain model into a relational database using object-relational mapping (ORM), which is the basis for JPA. In effect, ORM specifies how sets of Java objects, including references between them, are mapped to rows and columns in database tables. The first part of this chapter briefly discusses the difference between the object-oriented and relational world, also known as impedance mismatch. Later sections of the chapter explore the ORM features of the EJB 3 JPA. If you are a seasoned enterprise developer, you are probably comfortable with relational databases. If this is not the case, then refer to appendix B for a primer on some relatively obscure relational database concepts such as normalization and sequence columns that you must grasp to have a clear understanding of the intricacies of O/R mapping. We start our discussion by taking a look at the basic motivation behind O/R mapping, the so-called impedance mismatch. Then we ll begin our analysis by mapping domain objects, and move on to mapping relations. Finally we ll examine the concept of map inheritance and you ll learn about the inheritance strategies supported by JPA.

5. 6.

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

remove the default throw new code and add the code that will only proceed if there is no error: void xmlClient_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e { // if e.error is null we can proceed. if (e.Error == null) { } }

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-

void xmlClient_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e) { // if e.error is null we can proceed. if (e.Error == null) { string xmlData = e.Result; MessageBox.Show(xmlData); } }

ciations; you just define the relationship mapping between the superclass and the associated entity and the association becomes polymorphic.

Select the Form se ttings link to configure settings such as the confirmation message, redirect URL, submission limit, roles that can submit the form, and advanced settings, as shown in Figure 7-35.

8.5 Summary

Now, if you press F5 to compile and run the application, you should see something like what s shown in Figure 8-10.