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Single-valued entity associations
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A shipment has an optional link with a single auction item.
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In the database schema, you add an intermediate link table called ITEM_SHIPMENT. A row in this table represents a Shipment made in the context of an auction. The tables are shown in figure 7.5. You now map two classes to three tables: first in XML, and then with annotations. Mapping a join table in XML The property that represents the association from Shipment to Item is called auction:
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public class Shipment { ... private Item auction; ... // Getter/setter methods }
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Because you have to map this association with a foreign key column, you need the <many-to-one> mapping element in XML. However, the foreign key column isn t in the SHIPMENT table, it s in the ITEM_SHIPMENT join table. With the help of the <join> mapping element, you move it there.
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An optional one-to-many relationship mapped to a join table
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<class name="Shipment" table="SHIPMENT"> <id name="id" column="SHIPMENT_ID">...</id> ... <join table="ITEM_SHIPMENT" optional="true"> <key column="SHIPMENT_ID"/> <many-to-one name="auction" column="ITEM_ID" not-null="true" unique="true"/> </join> </class>
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The join table has two foreign key columns: SHIPMENT_ID, referencing the primary key of the SHIPMENT table; and ITEM_ID, referencing the ITEM table. The ITEM_ID column is unique; a particular item can be assigned to exactly one shipment. Because the primary key of the join table is SHIPMENT_ID, which makes this column also unique, you have a guaranteed one-to-one multiplicity between Shipment and Item. By setting optional="true" on the <join> mapping, you tell Hibernate that it should insert a row into the join table only if the properties grouped by this mapping are non-null. But if a row needs to be inserted (because you called aShipment.setAuction(anItem)), the NOT NULL constraint on the ITEM_ID column applies. You could map this association bidirectional, with the same technique on the other side. However, optional one-to-one associations are unidirectional most of the time. JPA also supports association join tables as secondary tables for an entity. Mapping secondary join tables with annotations You can map an optional one-to-one association to an intermediate join table with annotations:
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public class Shipment { @OneToOne @JoinTable( name="ITEM_SHIPMENT", joinColumns = @JoinColumn(name = "SHIPMENT_ID"), inverseJoinColumns = @JoinColumn(name = "ITEM_ID") ) private Item auction; ... // Getter/setter methods }
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Single-valued entity associations
You don t have to specify the SHIPMENT_ID column because it s automatically considered to be the join column; it s the primary key column of the SHIPMENT table. Alternatively, you can map properties of a JPA entity to more than one table, as demonstrated in Moving properties into a secondary table in chapter 8, section 8.1.3. First, you need to declare the secondary table for the entity:
@Entity @Table(name = "SHIPMENT") @SecondaryTable(name = "ITEM_SHIPMENT") public class Shipment { @Id @GeneratedValue @Column(name = "SHIPMENT_ID") private Long id; ... }
Note that the @SecondaryTable annotation also supports attributes to declare the foreign-key column name the equivalent of the <key column="..."/> you saw earlier in XML and the joinColumn(s) in a @JoinTable. If you don t specify it, the primary-key column name of the entity is used in this case, again SHIPMENT_ID. The auction property mapping is a @OneToOne; and as before, the foreign key column referencing the ITEM table is moved to the intermediate secondary table:
... public class Shipment { ... @OneToOne @JoinColumn(table = "ITEM_SHIPMENT", name = "ITEM_ID") private Item auction; }
The table for the target @JoinColumn is named explicitly. Why would you use this approach instead of the (simpler) @JoinTable strategy Declaring a secondary table for an entity is useful if not only one property (the many-to-one in this case) but several properties must be moved into the secondary table. We don t have a great example with Shipment and Item, but if your ITEM_SHIPMENT table would have additional columns, mapping these columns to properties of the Shipment entity might be useful. This completes our discussion of one-to-one association mappings. To summarize, use a shared primary key association if one of the two entities seems more important and can act as the primary key source. Use a foreign key association in all other cases, and a hidden intermediate join table when your one-to-one association is optional.
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