data matrix code java generator Many-to-many relationship between Category and Items in Java

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Listing 7.9 Many-to-many relationship between Category and Items
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@Entity public class Category {
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Implementing domain models
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@Id protected Long categoryId; protected String name; ... @ManyToMany protected Set<Item> items; ... } @Entity public class Item { @Id protected Long itemId; Subordinate protected String title; many-to-many ... relationship @ManyToMany(mappedBy="items") protected Set<Category> categories; ... }
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Owning many-to-many relationship
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In listing 7.9, the Category object s items variable is marked by the @ManyToMany annotation and is the owning side of the bidirectional association. In contrast, the Item object s categories variable signifies the subordinate bidirectional many-tomany association. As in the case of one-to-many relationships, the @ManyToMany annotation is missing the optional attribute. This is because an empty Set or List implicitly means an optional relationship, meaning that the entity can exist even if no associations do. As a handy reference, we summarize the various elements available in the @OneToOne, @OneToMany, @ManyToOne, and @ManyToMany annotations in table 7.3.
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Table 7.3 Elements available in the @OneToOne, @OneToMany, @ManyToOne, and @ManyToMany annotations Element targetEntity cascade fetch optional mappedBy @OneToOne Yes Yes Yes Yes Yes @OneToMany Yes Yes Yes No Yes @ManyToOne Yes Yes Yes Yes No @ManyToMany Yes Yes Yes No Yes
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Summary
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RIP: container-managed relationships
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If you have used EJB 2, you might be familiar with the container-managed relationship (CMR) feature of entity beans with bidirectional relationships. This feature monitored changes on either side of the relationship and updated the other side automatically. CMR is not supported in this version because entities can possibly be used outside of containers. However, mimicking this feature is not too hard using a few extra lines of code. Let us take the User-BillingInfo one-to-one relationship, for example. The code for changing the BillingInfo object for a User and making sure both sides of the relationship are still accurate would look like this:
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user.setBilling(billing); billing.setUser(user);
7.4 Summary
In this chapter, we discussed basic domain modeling concepts: entities, relationships, and how to define them using JPA. The lightweight API makes creating rich, elegant, object-oriented domain models a simple matter of applying annotations or deployment descriptor settings to plain Java objects. The even greater departure from the heavyweight, framework code laden approach of EJB 2 is the fact that the new persistence API can be separated altogether from the container, as you ll see in upcoming chapters. It is interesting to note that the API doesn t directly control relationship multiplicity. In the case of one-to-one and many-to-one relationships, the optional element somewhat specifies the multiplicity of the relationship. However, in the case of one-to-many and many-to-many relationships, the API does not enforce multiplicity at all. It is instead the responsibility of the programmer to control the size of the collection holding entity references (java.util.Set objects in our examples) and therefore control multiplicity. In chapter 8 we move on to the next step in building the ActionBazaar persistence layer and show you how to map the entities and relationships we created to the database using object-relational mapping.
Object-relational mapping
This chapter covers
Object-relational mapping concepts Mapping entities to tables Relationship mapping Inheritance mapping strategies
The impedance mismatch
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.
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