c# print document barcode Figure 6.8 Relationship between Item and Bid in Java

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Figure 6.8 Relationship between Item and Bid
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Mapping a parent/children relationship
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POJO-oriented persistence engines such as Hibernate don t implement managed associations, and POJO standards such as EJB 3.0 and Java Persistence don t require managed associations. Contrary to EJB 2.0 CMR, Hibernate and JPA associ-
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ations are all inherently unidirectional. As far as Hibernate is concerned, the association from Bid to Item is a different association than the association from Item to Bid! This is a good thing otherwise your entity classes wouldn t be usable outside of a runtime container (CMR was a major reason why EJB 2.1 entities were considered problematic). Because associations are so important, you need a precise language for classifying them.
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Multiplicity
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In describing and classifying associations, we ll almost always use the term multiplicity. In our example, the multiplicity is just two bits of information:
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Can there be more than one Bid for a particular Item Can there be more than one Item for a particular Bid
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After glancing at the domain model (see figure 6.8), you can conclude that the association from Bid to Item is a many-to-one association. Recalling that associations are directional, you classify the inverse association from Item to Bid as a oneto-many association. There are only two more possibilities: many-to-many and one-to-one. We ll get back to these in the next chapter. In the context of object persistence, we aren t interested in whether many means two or a maximum of five or unrestricted. And we re only barely interested in optionality of most associations; we don t especially care whether an associated instance is required or if the other end in an association can be NULL (meaning zero-to-many and to-zero association) However, these are important aspects in your relational data schema that influence your choice of integrity rules and the constraints you define in SQL DDL (see chapter 8, section 8.3, Improving schema DDL ).
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The simplest possible association
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The association from Bid to Item (and vice versa) is an example of the simplest possible kind of entity association. You have two properties in two classes. One is a collection of references, and the other a single reference. First, here s the Java class implementation of Bid:
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Mapping collections and entity associations
public class Bid { ... private Item item; public void setItem(Item item) { this.item = item; } public Item getItem() { return item; } ... }
Next, this is the Hibernate mapping for this association:
<class name="Bid" table="BID"> ... <many-to-one name="item" column="ITEM_ID" class="Item" not-null="true"/> </class>
This mapping is called a unidirectional many-to-one association. (Actually, because it s unidirectional, you don t know what is on the other side, and you could just as well call this mapping a unidirectional to-one association mapping.) The column ITEM_ID in the BID table is a foreign key to the primary key of the ITEM table. You name the class Item, which is the target of this association, explicitly. This is usually optional, because Hibernate can determine the target type with reflection on the Java property. You added the not-null attribute because you can t have a bid without an item a constraint is generated in the SQL DDL to reflect this. The foreign key column ITEM_ID in the BID can never be NULL, the association is not to-zero-orone. The table structure for this association mapping is shown in figure 6.9.
Figure 6.9 Table relationships and keys for a one-to-many mapping
Mapping a parent/children relationship
In JPA, you map this association with the @ManyToOne annotation, either on the field or getter method, depending on the access strategy for the entity (determined by the position of the @Id annotation):
public class Bid { ... @ManyToOne( targetEntity = auction.model.Item.class ) @JoinColumn(name = "ITEM_ID", nullable = false) private Item item; ... }
There are two optional elements in this mapping. First, you don t have to include the targetEntity of the association; it s implicit from the type of the field. An explicit targetEntity attribute is useful in more complex domain models for example, when you map a @ManyToOne on a getter method that returns a delegate class, which mimics a particular target entity interface. The second optional element is the @JoinColumn. If you don t declare the name of the foreign key column, Hibernate automatically uses a combination of the target entity name and the database identifier property name of the target entity. In other words, if you don t add a @JoinColumn annotation, the default name for the foreign key column is item plus id, separated with an underscore. However, because you want to make the foreign key column NOT NULL, you need the annotation anyway to set nullable = false. If you generate the schema with the Hibernate Tools, the optional="false" attribute on the @ManyToOne would also result in a NOT NULL constraint on the generated column. This was easy. It s critically important to realize that you can write a complete application without using anything else. (Well, maybe a shared primary key oneto-one mapping from time to time, as shown in the next chapter.) You don t need to map the other side of this class association, and you ve already mapped everything present in the SQL schema (the foreign key column). If you need the Item instance for which a particular Bid was made, call aBid.getItem(), utilizing the entity association you created. On the other hand, if you need all bids that have been made for an item, you can write a query (in whatever language Hibernate supports). One of the reasons you use a full object/relational mapping tool like Hibernate is, of course, that you don t want to write that query.
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