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EclipseLink/UserGuide/JPA/Basic JPA Development/Entities/Inheritance

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EclipseLink JPA

@Inheritance

You can use the @Inheritance annotation or <inheritance> XML element to configure how entities with inheritance are persisted.


By default the secondary table is joined to the primary table's id columns. This assumes that the secondary table has the same id columns defined with the same name. If the secondary table uses different names for the id columns, then a set or @PrimaryKeyJoinColumns must be provided. @PrimaryKeyJoinColumn can also be used for JOINED inheritance to specify how the child table is joined to the parent table. @PrimaryKeyJoinColumn still requires that the secondary table have all of the id columns of the primary table, for more advanced multiple table joins see Advanced Multiple Table Configuration.

@Inheritance Attributes
Attribute Description Default Required?
name The name of the table Yes
catalog A String catalog name. Default catalog for database No
schema The String name of the schema. Default schema of the database No
uniqueConstraints No additional constraints No
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For more information, see Section 11.1.xx "Inheritance Annotation" in the JPA Specification.

@DiscriminatorColumn Attributes
Attribute Description Default Required?
name The name foreign key column in the secondary table that references the id column in the primary table id column No
referencedColumnName A name of the id column in the primary table that is being referenced. This is only required for composite ids. id column No
columnDefinition Optional column description for use with DDL generation. No
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For more information, see Section 11.1.xx "DiscriminatorColumn Annotation" in the JPA Specification.

The following example shows how to use this annotation to specify the table for Employee.

Example: Using @Inheritance
@Entity
@Table(name="EMP")
@SecondaryTable(name="SALARY")
public class Employee implements Serializable {
    ...
    @Id
    public Long getId() {
        return id;
    }
    ...
}
Example: Using <inheritance>
<entity class="Employee">
    <table name="EMP"/>
    <secondary-table name="SALARY"/>
    <attributes>
        <id name="id"/>
        ...
    </attributes>
</entity>


Advanced Inheritance Configuration

JPA requires that secondary tables contain the id of the entity. EclipseLink allows for the multiple table join to be based on foreign keys, or on specific criteria.

To define a secondary table that is joined through a foreign key, the ClassDescriptor.addForeignKeyFieldNameForMultipleTable() API is used. This column name given to this API should be prefixed by its table name, this allows for foreign keys from any of the secondary tables to be used in the multiple table join. Note that all of the column values still need to be mapped at least once.

If the multiple table join is more complex, an EclipseLink Expression can be used to define the join. The Expression is set on the ClassDescriptor's query manager using the DescriptorQueryManager.setMultipleTableJoinExpression() API. It is even possible to define a join Expression that make use of an outer-join on certain databases.

In certain advanced cases multiple tables can be encapsulated using a database view. This allows for any table join to be define in SQL and for the entity to be simply mapped to the view. To map an entity to a view, just use the view name instead of the table name. If the view is not updateable, your database may support instead of triggers to allow updates.

Example: Using a DescriptorCustomizer to configure a secondary table joined through a foreign key
@Entity
@Table(name="EMP")
@SecondaryTable(name="EMP_INFO")
@Customizer(EmployeeCustomizer.class)
public class Employee implements Serializable {
    @Id
    private Long id;
    @Basic
    @Column(name="ID", table="EMP_INFO")
    private String infoId;
    ...
}
public class EmployeeCustomizer implements DescriptorCustomizer {
    public void customize(ClassDescriptor descriptor) {
        descriptor.getAdditionalTablePrimaryKeyFields().clear();
        descriptor.getMultipleTableForeignKeys()().clear();
        descriptor.addForeignKeyFieldNameForMultipleTable("EMP.INFO_ID", "EMP_INFO.ID");
    }
}
Example: Using a DescriptorCustomizer to configure a secondary table joined through an Expression
@Entity
@Table(name="EMP")
@SecondaryTable(name="EMP_INFO")
@Customizer(EmployeeCustomizer.class)
public class Employee implements Serializable {
    @Id
    private Long id;
    @Basic
    @Column(name="ID", table="EMP_INFO")
    private String infoId;
    ...
}
public class EmployeeCustomizer implements DescriptorCustomizer {
    public void customize(ClassDescriptor descriptor) {
        ExpressionBuilder emp = new ExpressionBuilder();
        Expression join = emp.getField("EMP.INFO_ID").equal(emp.getField("EMP_INFO.ID").and(emp.getField("EMP_INFO.ARCHIVE").equal(false));
        descriptor.getQueryManager().setMultipleTableJoinExpression(join);
    }
}

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