Difference between revisions of "EclipseLink/UserGuide/JPA/Basic JPA Development/Entities/Inheritance"

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(@Inheritance)
(Advanced Inheritance Configuration)
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==Advanced Inheritance Configuration==
 
==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 <code>ClassDescriptor.addForeignKeyFieldNameForMultipleTable()</code> API is usedThis 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 joinNote that all of the column values still need to be mapped at least once.
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===@ClassExtractor===
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If you are mapping to an existing database, and the tables do not have a discriminator column you can still define inheritance using the EclipseLink <code>@ClassExtractor</code> or <code><nowiki>class-extractor</no-wiki></code> XML element.  The class extractor takes a class that implements the <code>ClassExtractor</code> interfaceEclipseLink uses a instance of this class to determine the class type to use for a database rowThe class extractor must define a <code>extratClass</code> method that takes the database <code>Record</code> and EclipseLink <code>Session</code>.
  
If the multiple table join is more complex, an EclipseLink <code>Expression</code> can be used to define the joinThe <code>Expression</code> is set on the <code>ClassDescriptor</code>'s query manager using the <code>DescriptorQueryManager.setMultipleTableJoinExpression()</code> API.  It is even possible to define a join <code>Expression</code> that make use of an outer-join on certain databases.
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If a class extractor is used with <code>SINGLE_TABLE</code> inheritance, EclipseLink needs to be able to filter the rows of the class type in queriesThis can be accomplished in EclipseLink through setting an <code>onlyInstancesExpression</code> or <code>withAllSubclassesExpression</code> for branch classes.  These can be set to EclipseLink <code>Expression</code> objects using a <code>DescriptorCustomizer</code>.
  
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.
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======''Example: Using ClassExtractor do define inheritance''======
 
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======''Example: Using a DescriptorCustomizer to configure a secondary table joined through a foreign key''======
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<source lang="java">
 
<source lang="java">
 
@Entity
 
@Entity

Revision as of 13:39, 23 June 2011

EclipseLink JPA

Contents


@Inheritance

You can use the @Inheritance annotation or <inheritance> XML element to configure how entities with inheritance are persisted. JPA defines three inheritance strategies SINGLE_TABLE, JOINED, and TABLE_PER_CLASS. By default the SINGLE_TABLE strategy is used and all of the subclasses are persisted in a single table that contains all of the column of all of the subclasses. In addition a discriminator column named DTYPE is required in the table to store the class type.

For other inheritance strategies see Advanced Inheritance Configuration.

The @Inheritance annotation has the following attributes:

  • strategy – By default, the EclipseLink persistence provider assumes that all the classes in a hierarchy are mapped to a single table differentiated by the discriminator value (see @DiscriminatorValue) in the table's discriminator column (see @DiscriminatorColumn): InheritanceType.SINGLE_TABLE.
    If this is not appropriate for your application or if you must match an existing data model, set strategy to the desired InheritanceType enumerated type:
    • SINGLE_TABLE – all the classes in a hierarchy are mapped to a single table. The table has a discriminator column (@DiscriminatorColumn) whose value (@DiscriminatorValue) identifies the specific subclass to which the instance that is represented by the row belongs.

      Elug note icon.png

      Note: This option provides the best support for both polymorphic relationships between entities and queries that range over the class hierarchy. The disadvantages of this option include the need to make nullable columns that should be NOT NULL.


    • TABLE_PER_CLASS – each class is mapped to a separate table. All properties of the class, including inherited properties, are mapped to columns of the table for the class.

      Elug note icon.png

      Note: This option has several limitations when querying or having relationships to the root or branch classes. Joins to root or branch classes are not supported.

    • JOINED – the root of the class hierarchy is represented by a single table and each subclass is represented by a separate table. Each subclass table contains only those fields that are specific to the subclass (not inherited from its superclass) and primary key columns that serve as foreign keys to the primary keys of the superclass table. The join from the primary table to the subclass table can be configured using the @PrimaryKeyJoinColumn, see SecondaryTable for more info. If you have multiple levels of inheritance, each subclass table should join with the root table, and the discriminator column is only defined once in the root table.
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For more information, see Section 2.1.9 "Inheritance" in the JPA Specification.

Elug javaspec icon.gif

For more information, see Section 9.1.29 "Inheritance Annotation" in the JPA Specification.

@DiscriminatorColumn

You can use the @DiscriminatorColumn annotation or <discriminator-column> XML element to configure the name or type of the inheritance discriminator column. The discriminator column is required for SINGLE_TABLE and JOINED inheritance and stores the associated entity type for the row. The default name for the discriminator column is DTYPE. JPA only allows String or Integer values for discriminators. Through the EclipseLink API, it is possible to use other discriminator types, and it is possible to not have a discriminator, or use custom discriminator, see Advanced Inheritance Configuration.

@DiscriminatorColumn Attributes
Attribute Description Default Required?
name The name of column to be used to store the class discriminator value. DTYPE No
discriminatorType The type of the discriminator value, defined in DiscriminatorType, one of STRING, INTEGER, and CHAR. STRING No
columnDefinition Optional column description for use with DDL generation. generated base on discriminatorType No
length The size of the column for DDL generation. Only relevant for STRING types. 31 No
Elug javaspec icon.gif

For more information, see Section 11.1.10 "DiscriminatorColumn Annotation" in the JPA Specification.

@DiscriminatorValue

You can use the @DiscriminatorValue annotation or <discriminator-value> XML element to configure the value of the inheritance discriminator. The discriminator value can be specified in each non-abstract class in the inheritance hierarchy. By default the discriminator value is the entity's name, which defaults to its unprefixed class name. The discriminator value is always specified as a String, but is converted to the discriminator column type.


The following examples shows usages of the three different inheritance strategies for mapping an Account hierarchy.

Example: Using SINGLE_TABLE with @Inheritance annotation
CREATE TABLE ACCOUNT (ID NUMBER, ACCOUNT_TYPE VARCHAR(31), BALANCE NUMBER, INTERESTRATE NUMBER, RETURNCHECKS BOOLEAN)
@Entity
@Table(name="ACCOUNT")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="ACCOUNT_TYPE")
public abstract class Account implements Serializable {
    @Id
    private Long id;
    @Basic
    private BigDecimal balance;
    ...
}
@Entity
@DiscriminatorValue("SAVINGS")
public class SavingAccount implements Serializable {
    @Basic
    private BigDecimal interestRate;
}
@Entity
@DiscriminatorValue("CHECKING")
public class CheckingAccount implements Serializable {
    @Basic
    private boolean returnChecks;
}
Example: Using SINGLE_TABLE with <inheritance> XML
<entity class="Account">
    <table name="ACCOUNT"/>
    <inheritance strategy="SINGLE_TABLE"/>
    <discriminator-column name="ACCOUNT_TYPE"/>
    <attributes>
        <id name="id"/>
        <basic name="balance"/>
    </attributes>
</entity>
<entity class="SavingAccount">
    <discriminator-value>SAVINGS</discriminator-value>
    <attributes>
        <basic name="interestRate"/>
    </attributes>
</entity>
<entity class="CheckingAccount">
    <discriminator-value>CHECKING</discriminator-value>
    <attributes>
        <basic name="returnChecks"/>
    </attributes>
</entity>


Example: Using JOINED with @Inheritance annotation
CREATE TABLE ACCOUNT (ID NUMBER, ACCOUNT_TYPE VARCHAR(31), BALANCE NUMBER)
CREATE TABLE SAVING_ACCOUNT (ID NUMBER, INTERESTRATE NUMBER)
CREATE TABLE CHECKING_ACCOUNT (ACCOUNT_ID NUMBER, RETURNCHECKS BOOLEAN)
@Entity
@Table(name="ACCOUNT")
@Inheritance(strategy=InheritanceType.JOINED)
// This example shows usage of integer discriminators.
@DiscriminatorColumn(name="ACCOUNT_TYPE", discriminatorType=DiscriminatorType.INTEGER)
public abstract class Account implements Serializable {
    @Id
    private Long id;
    @Basic
    private BigDecimal balance;
    ...
}
@Entity
@Table(name="SAVING_ACCOUNT")
@DiscriminatorValue("1")
public class SavingAccount implements Serializable {
    @Basic
    private BigDecimal interestRate;
}
@Entity
@Table(name="CHECKING_ACCOUNT")
// This example shows usage of a join column as the id column is not the default name.
@PrimaryKeyJoinColumn(name="ACCOUNT_ID" referencedColumnName="ID")
@DiscriminatorValue("2")
public class CheckingAccount implements Serializable {
    @Basic
    private boolean returnChecks;
}
Example: Using JOINED with <inheritance> XML
<entity class="Account">
    <table name="ACCOUNT"/>
    <inheritance strategy="JOINED"/>
    <discriminator-column name="ACCOUNT_TYPE" discriminator-type="INTEGER"/>
    <attributes>
        <id name="id"/>
        <basic name="balance"/>
    </attributes>
</entity>
<entity class="SavingAccount">
    <table name="SAVINGS_ACCOUNT"/>
    <discriminator-value>SAVINGS</discriminator-value>
    <attributes>
        <basic name="interestRate"/>
    </attributes>
</entity>
<entity class="CheckingAccount">
    <table name="CHECKING_ACCOUNT"/>
    <primary-key-join-column name="ACCOUNT_ID" referenced-column-name="ID"/>
    <discriminator-value>CHECKING</discriminator-value>
    <attributes>
        <basic name="returnChecks"/>
    </attributes>
</entity>


Example: Using TABLE_PER_CLASS with @Inheritance annotation
CREATE TABLE SAVINGS_ACCOUNT (ID NUMBER, BALANCE NUMBER, INTERESTRATE NUMBER)
CREATE TABLE CHECKING_ACCOUNT (ID NUMBER, BALANCE NUMBER, RETURNCHECKS BOOLEAN)
@Entity
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
public abstract class Account implements Serializable {
    @Id
    private Long id;
    @Basic
    private BigDecimal balance;
    ...
}
@Entity
@Table(name="SAVINGS_ACCOUNT")
public class SavingAccount implements Serializable {
    @Basic
    private BigDecimal interestRate;
}
@Entity
@Table(name="CHECKING_ACCOUNT")
public class CheckingAccount implements Serializable {
    @Basic
    private boolean returnChecks;
}
Example: Using TABLE_PER_CLASS with <inheritance> XML
<entity class="Account">
    <inheritance strategy="TABLE_PER_CLASS"/>
    <attributes>
        <id name="id"/>
        <basic name="balance"/>
    </attributes>
</entity>
<entity class="SavingAccount">
    <table>SAVINGS_ACCOUNT</table>
    <attributes>
        <basic name="interestRate"/>
    </attributes>
</entity>
<entity class="CheckingAccount">
    <table>CHECKING_ACCOUNT</table>
    <attributes>
        <basic name="returnChecks"/>
    </attributes>
</entity>

Advanced Inheritance Configuration

@ClassExtractor

If you are mapping to an existing database, and the tables do not have a discriminator column you can still define inheritance using the EclipseLink @ClassExtractor or class-extractor</no-wiki></code> XML element. The class extractor takes a class that implements the <code>ClassExtractor</code> interface. EclipseLink uses a instance of this class to determine the class type to use for a database row. The class extractor must define a <code>extratClass</code> method that takes the database <code>Record</code> and EclipseLink <code>Session</code>. If a class extractor is used with <code>SINGLE_TABLE</code> inheritance, EclipseLink needs to be able to filter the rows of the class type in queries. This can be accomplished in EclipseLink through setting an <code>onlyInstancesExpression</code> or <code>withAllSubclassesExpression</code> for branch classes. These can be set to EclipseLink <code>Expression</code> objects using a <code>DescriptorCustomizer</code>. ======''Example: Using ClassExtractor do define inheritance''====== <source lang="java"> @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; ... } </source> <source lang="java"> public class EmployeeCustomizer implements DescriptorCustomizer { public void customize(ClassDescriptor descriptor) { descriptor.getAdditionalTablePrimaryKeyFields().clear(); descriptor.getMultipleTableForeignKeys()().clear(); descriptor.addForeignKeyFieldNameForMultipleTable("EMP.INFO_ID", "EMP_INFO.ID"); } } </source> ======''Example: Using a DescriptorCustomizer to configure a secondary table joined through an Expression''====== <source lang="java"> @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; ... } </source> <source lang="java"> 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); } } </source> {{EclipseLink_JPA |previous=[[EclipseLink/UserGuide/JPA/Basic JPA Development/Entities/SecondaryTable|@SecondaryTable]] |next= [[EclipseLink/UserGuide/JPA/Basic JPA Development/Mapping|Configuring Mappings]] |up= [[EclipseLink/UserGuide/JPA/Basic JPA Development/Entities/Creating_and_Configuring_Entities|Configuring Entities]] |version=2.2.0 DRAFT}}