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

(Allowing Zero Value Primary Keys)
 
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Use the <tt>@Id</tt> annotation or <code><id></code> XML element to designate one or more persistent fields or properties as the entity's primary key.
 
Use the <tt>@Id</tt> annotation or <code><id></code> XML element to designate one or more persistent fields or properties as the entity's primary key.
  
For each entity, you must designate at least one of the following:
+
For each entity, you must designate one of the following:
 
* one <tt>@Id</tt>, or
 
* one <tt>@Id</tt>, or
 
* one <tt>[[EclipseLink/UserGuide/JPA/Basic JPA Development/Mapping/Entity/EmbeddedId|@EmbeddedId]]</tt>, or
 
* one <tt>[[EclipseLink/UserGuide/JPA/Basic JPA Development/Mapping/Entity/EmbeddedId|@EmbeddedId]]</tt>, or
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If the <code>Id</code> (or part of the Id) is also a foreign key from a reference to another entity, the <tt>@Id</tt> annotation should be placed on the <tt>@OneToOne</tt> or <tt>@ManyToOne</tt> attribute.  The entity's <code>Id</code> is then composed of the <code>Id</code> of the referenced entity.
 
If the <code>Id</code> (or part of the Id) is also a foreign key from a reference to another entity, the <tt>@Id</tt> annotation should be placed on the <tt>@OneToOne</tt> or <tt>@ManyToOne</tt> attribute.  The entity's <code>Id</code> is then composed of the <code>Id</code> of the referenced entity.
 +
 +
{{EclipseLink_Note
 +
|note=The id of a persistent object instance must never be changed and cannot be updated.
 +
}}
  
 
This example shows how to use this annotation to designate the persistent field <tt>empID</tt> as the primary key of the <tt>Employee</tt> table.
 
This example shows how to use this annotation to designate the persistent field <tt>empID</tt> as the primary key of the <tt>Employee</tt> table.
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It is possible to generate the value for an <code>Id</code> using the <code>prePersist</code> event, or through triggers when using the Oracle database platform.  See [[EclipseLink/UserGuide/JPA/Advanced_JPA_Development/Returning|Returning]].
 
It is possible to generate the value for an <code>Id</code> using the <code>prePersist</code> event, or through triggers when using the Oracle database platform.  See [[EclipseLink/UserGuide/JPA/Advanced_JPA_Development/Returning|Returning]].
  
===Allowing Zero Value Primary Keys===
+
==Allowing Zero Value Primary Keys==
By default, EclipseLink interprets zero as ''null'' for primitive types that cannot be null (such as <tt>int</tt> and <tt>long</tt>) causing zero to be an invalid value for primary keys. You can modify this setting by using the '''eclipselink.id-validation''' property in the <tt>persistence.xml file</tt>. Valid values are:
+
By default, EclipseLink interprets zero as ''null'' for primitive types that cannot be null (such as <tt>int</tt> and <tt>long</tt>) causing zero to be an invalid value for primary keys. You can modify this setting by using the '''eclipselink.id-validation''' property in the <tt>persistence.xml file</tt>, or by using the <code>@PrimaryKey</code> annotation to configure an <code>IdValidation</code> for an entity class.<br/>
 +
Valid values are defined in the <code>IdValidation</code> enum:
 
*'''NULL''' – EclipseLink interprets zero values as ''zero''. This permits primary keys to use a value of zero.
 
*'''NULL''' – EclipseLink interprets zero values as ''zero''. This permits primary keys to use a value of zero.
 
*'''ZERO''' (default) – EclipseLink interprets zero as ''null''.
 
*'''ZERO''' (default) – EclipseLink interprets zero as ''null''.

Latest revision as of 12:33, 20 June 2011

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@Id

Use the @Id annotation or <id> XML element to designate one or more persistent fields or properties as the entity's primary key.

For each entity, you must designate one of the following:

Elug note icon.png

Note: The @Id and @IdClass combination – is only applicable to composite primary key configuration.


The @Id annotation does not have attributes.

By default, the entities Id must be set by the application, normally before the persist is called. A @GeneratedValue can be used to have EclipseLink generate the Id value.

If the Id (or part of the Id) is also a foreign key from a reference to another entity, the @Id annotation should be placed on the @OneToOne or @ManyToOne attribute. The entity's Id is then composed of the Id of the referenced entity.

Elug note icon.png

Note: The id of a persistent object instance must never be changed and cannot be updated.

This example shows how to use this annotation to designate the persistent field empID as the primary key of the Employee table.

Example: @Id Annotation
@Entity
public class Employee implements Serializable {
    @Id
    private int empID;
    ...
}

Example: Id XML
<entity class="Employee">
    <attributes>
        <id name="empID"/>
        ...
    </attributes>
</entity>
Elug javaspec icon.gif

For more information, see Section 11.1.18 "Id Annotation" in the JPA Specification.

The @Id annotation supports the use of EclipseLink converters. See Using EclipseLink JPA Converters.

It is possible to generate the value for an Id using the prePersist event, or through triggers when using the Oracle database platform. See Returning.

Allowing Zero Value Primary Keys

By default, EclipseLink interprets zero as null for primitive types that cannot be null (such as int and long) causing zero to be an invalid value for primary keys. You can modify this setting by using the eclipselink.id-validation property in the persistence.xml file, or by using the @PrimaryKey annotation to configure an IdValidation for an entity class.
Valid values are defined in the IdValidation enum:

  • NULL – EclipseLink interprets zero values as zero. This permits primary keys to use a value of zero.
  • ZERO (default) – EclipseLink interprets zero as null.
  • NEGATIVE – EclipseLink interprets negative values as null.
  • NONE – EclipseLink does not validate the id value.

You can also use the @PrimaryKey annotation to configure the id validation for a specific entity.


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