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Legacy databases and custom SQL
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<many-to-many> mapping to create a literal join condition from the association
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table to the entity table(s). Unfortunately, at the time of writing, Hibernate Annotations doesn t support arbitrary join conditions expressed with formulas. The grouping of properties under a reference name also wasn t possible. We expect that these features will closely resemble the XML mapping, once they re available. Another issue you may encounter in a legacy schema is that it doesn t integrate nicely with your class granularity. Our usual recommendation to have more classes than tables may not work, and you may have to do the opposite and join arbitrary tables into one class.
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Joining arbitrary tables
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We ve already shown the <join> mapping element in an inheritance mapping in chapter 5; see section 5.1.5, Mixing inheritance strategies. It helped to break out properties of a particular subclass into a separate table, out of the primary inheritance hierarchy table. This generic functionality has more uses however, we have to warn you that <join> can also be a bad idea. Any properly designed system should have more classes than tables. Splitting a single class into separate tables is something you should do only when you need to merge several tables in a legacy schema into a single class. Moving properties into a secondary table Suppose that in CaveatEmptor, you aren t keeping a user s address information with the user s main information in the USERS table, mapped as a component, but in a separate table. This is shown in figure 8.4. Note that each BILLING_ADDRESS has a foreign key USER_ID, which is in turn the primary key of the BILLING_ ADDRESS table. To map this in XML, you need to group the properties of the Address in a <join> element:
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Figure 8.4 Breaking out the billing address data into a secondary table
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<class name="User" table="USERS"> <id>... <join table="BILLING_ADDRESS" optional="true"> <key column="USER_ID"/> <component name="billingAddress" class="Address"> <property name="street" type="string" column="STREET" length="255"/> <property name="zipcode" type="string" column="ZIPCODE" length="16"/> <property name="city" type="string" column="CITY" length="255"/> </component> </join> </class>
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You don t have to join a component; you can as well join individual properties or even a <many-to-one> (we did this in the previous chapter for optional entity associations). By setting optional="true", you indicate that the component property may also be null for a User with no billingAddress, and that no row should then be inserted into the secondary table. Hibernate also executes an outer join instead of an inner join to retrieve the row from the secondary table. If you declared fetch="select" on the <join> mapping, a secondary select would be used for that purpose. The notion of a secondary table is also included in the Java Persistence specification. First, you have to declare a secondary table (or several) for a particular entity:
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@Entity @Table(name = "USERS") @SecondaryTable( name = "BILLING_ADDRESS", pkJoinColumns = { @PrimaryKeyJoinColumn(name="USER_ID") } ) public class User { ... }
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Legacy databases and custom SQL
Each secondary table needs a name and a join condition. In this example, a foreign key column references the primary key column of the USERS table, just like earlier in the XML mapping. (This is the default join condition, so you can only declare the secondary table name, and nothing else). You can probably see that the syntax of annotations is starting to become an issue and code is more difficult to read. The good news is that you won t have to use secondary tables often. The actual component property, billingAddress, is mapped as a regular @Embedded class, just like a regular component. However, you need to override each component property column and assign it to the secondary table, in the User class:
@Embedded @AttributeOverrides( { @AttributeOverride( name = "street", column = @Column(name="STREET", table = "BILLING_ADDRESS") ), @AttributeOverride( name = "zipcode", column = @Column(name="ZIPCODE", table = "BILLING_ADDRESS") ), @AttributeOverride( name = "city", column = @Column(name="CITY", table = "BILLING_ADDRESS") ) }) private Address billingAddress;
This is no longer easily readable, but it s the price you pay for mapping flexibility with declarative metadata in annotations. Or, you can use a JPA XML descriptor:
<entity class="auction.model.User" access="FIELD"> <table name="USERS"/> <secondary-table name="BILLING_ADDRESS"> <primary-key-join-column referenced-column-name="USER_ID"/> </secondary-table> <attributes> ... <embedded name="billingAddress"> <attribute-override name="street"> <column name="STREET" table="BILLING_ADDRESS"/> </attribute-override> <attribute-override name="zipcode">
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