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Single-valued entity associations
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<class name="Address" table="ADDRESS"> <id name="id" column="ADDRESS_ID"> <generator class="foreign"> <param name="property">user</param> </generator> </id> ... <one-to-one name="user" class="User" constrained="true"/> </class>
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This mapping seems strange at first. Read it as follows: When an Address is saved, the primary key value is taken from the user property. The user property is a reference to a User object; hence, the primary key value that is inserted is the same as the primary key value of that instance. Look at the table structure in figure 7.2.
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Figure 7.2 The USERS and ADDRESS tables have the same primary keys.
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The code to save both objects now has to consider the bidirectional relationship, and it finally works:
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User newUser = new User(); Address shippingAddress = new Address(); newUser.setShippingAddress(shippingAddress); shippingAddress.setUser(newUser); // Bidirectional session.save(newUser);
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Let s do the same with annotations. Shared primary key with annotations JPA supports one-to-one entity associations with the @OneToOne annotation. To map the association of shippingAddress in the User class as a shared primary key association, you also need the @PrimaryKeyJoinColumn annotation:
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@OneToOne @PrimaryKeyJoinColumn private Address shippingAddress;
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This is all that is needed to create a unidirectional one-to-one association on a shared primary key. Note that you need @PrimaryKeyJoinColumns (plural)
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Advanced entity association mappings
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instead if you map with composite primary keys. In a JPA XML descriptor, a one-toone mapping looks like this:
<entity-mappings> <entity class="auction.model.User" access="FIELD"> ... <one-to-one name="shippingAddress"> <primary-key-join-column/> </one-to-one> </entity> </entity-mappings>
The JPA specification doesn t include a standardized method to deal with the problem of shared primary key generation, which means you re responsible for setting the identifier value of an Address instance correctly before you save it (to the identifier value of the linked User instance). Hibernate has an extension annotation for custom identifier generators which you can use with the Address entity (just like in XML):
@Entity @Table(name = "ADDRESS") public class Address { @Id @GeneratedValue(generator = "myForeignGenerator") @org.hibernate.annotations.GenericGenerator( name = "myForeignGenerator", strategy = "foreign", parameters = @Parameter(name = "property", value = "user") ) @Column(name = "ADDRESS_ID") private Long id; ... private User user; }
Shared primary key one-to-one associations aren t uncommon but are relatively rare. In many schemas, a to-one association is represented with a foreign key field and a unique constraint.
One-to-one foreign key associations
Instead of sharing a primary key, two rows can have a foreign key relationship. One table has a foreign key column that references the primary key of the associated table. (The source and target of this foreign key constraint can even be the same table: This is called a self-referencing relationship.)
Single-valued entity associations
Let s change the mapping from a User to an Address. Instead of the shared primary key, you now add a SHIPPING_ADDRESS_ID column in the USERS table:
<class name="User" table="USERS"> <many-to-one name="shippingAddress" class="Address" column="SHIPPING_ADDRESS_ID" cascade="save-update" unique="true"/> </class>
The mapping element in XML for this association is <many-to-one> not <oneto-one>, as you might have expected. The reason is simple: You don t care what s on the target side of the association, so you can treat it like a to-one association without the many part. All you want is to express This entity has a property that is a reference to an instance of another entity and use a foreign key field to represent that relationship. The database schema for this mapping is shown in figure 7.3.
A one-to-one foreign key association between USERS and ADDRESS
An additional constraint enforces this relationship as a real one to one. By making the SHIPPING_ADDRESS_ID column unique, you declare that a particular address can be referenced by at most one user, as a shipping address. This isn t as strong as the guarantee from a shared primary key association, which allows a particular address to be referenced by at most one user, period. With several foreign key columns (let s say you also have unique HOME_ADDRESS_ID and BILLING_ADDRESS_ID), you can reference the same address target row several times. But in any case, two users can t share the same address for the same purpose. Let s make the association from User to Address bidirectional. Inverse property reference The last foreign key association was mapped from User to Address with <many-toone> and a unique constraint to guarantee the desired multiplicity. What mapping
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