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HHH-9998 - Continue documentation TLC - natural-id
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‎documentation/src/main/docbook/mappingGuide/en-US/chapters/natural_id/Natural_Id.xml‎

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version="5.0"
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xml:lang="en"
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xmlns="http://docbook.org/ns/docbook"
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>
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<title>Natural Ids</title>
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<para>
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* simple
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* composite
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* caching
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* apis
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Natural ids represent unique identifiers that naturally exist within your domain model. Even if
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a natural id does not make a good primary key, it still is useful to tell Hibernate about it.
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As we will see later, Hibernate provides a dedicated, efficient API for loading and entity by its natural-id
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much like it offers for loading by identifier (PK).
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</para>
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<section xml:id="naturalid-mapping">
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<title>Natural Id Mapping</title>
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<para>
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Natural ids are defined in terms of one or more persistent attributes.
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</para>
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<example>
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<title>Natural id using single basic attribute</title>
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<programlisting role="JAVA"><xi:include href="extras/SimpleBasicNaturalIdMapping.java" parse="text" /></programlisting>
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</example>
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<example>
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<title>Natural id using single embedded attribute</title>
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<programlisting role="JAVA"><xi:include href="extras/SimpleCompositeNaturalIdMapping.java" parse="text" /></programlisting>
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</example>
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<example>
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<title>Natural id using multiple persistent attributes</title>
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<programlisting role="JAVA"><xi:include href="extras/NonSimpleNaturalIdMapping.java" parse="text" /></programlisting>
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</example>
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</section>
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<section xml:id="naturalid-api">
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<title>Natural Id API</title>
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<para>
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As stated before, Hibernate provides an API for loading entities by natural id. This is represented by the
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<interfacename>org.hibernate.NaturalIdLoadAccess</interfacename> contract obtained via
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<methodname>Session#byNaturalId</methodname>. If the entity does not define a natural id, an exception
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will be thrown there.
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</para>
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<example>
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<title>Using NaturalIdLoadAccess</title>
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<programlisting role="JAVA"><xi:include href="extras/NaturalIdLoadAccessUsage.java" parse="text" /></programlisting>
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</example>
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<para>
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NaturalIdLoadAccess offers 2 distinct methods for obtaining the entity:
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<itemizedlist>
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<listitem>
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<para>
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<methodname>load</methodname> - obtains a reference to the entity, making sure
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that the entity state is initialized.
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</para>
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</listitem>
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<listitem>
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<para>
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<methodname>getReference</methodname> - obtains a reference to the entity. The state
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may or may not be initialized. If the entity is associated with the Session already,
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that reference (loaded or not) is returned; else if the entity supports proxy
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generation, an uninitialized proxy is generated and returned; otherwise
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the entity is loaded from the database and returned.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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NaturalIdLoadAccess also allows to request locking for the load. We might use that to load an
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entity by natural id and at the same time apply a pessimistic lock. For additional details on locking,
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see the <citetitle>Hibernate User Guide</citetitle>.
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</para>
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<para>
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We will discuss the last method available on NaturalIdLoadAccess
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(<methodname>setSynchronizationEnabled</methodname>) in <xref linkend="naturalid-mutability-caching"/>.
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</para>
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<para>
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Because the Company and PostalCarrier entities define "simple" natural ids, we also allow simplified
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access to load them based on the natural ids.
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</para>
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<example>
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<title>Using SimpleNaturalIdLoadAccess</title>
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<programlisting role="JAVA"><xi:include href="extras/SimpleNaturalIdLoadAccessUsage.java" parse="text" /></programlisting>
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</example>
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<para>
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Here we see the use of the <interfacename>org.hibernate.SimpleNaturalIdLoadAccess</interfacename>
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contract, obtained via <methodname>Session#bySimpleNaturalId</methodname>. SimpleNaturalIdLoadAccess is similar
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to NaturalIdLoadAccess except that it does not define the <methodname>using</methodname> method. Instead,
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because these "simple" natural ids are defined based on just one attribute we can directly pass the
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corresponding value of that natural id attribute directly to the <methodname>load</methodname>
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and <methodname>getReference</methodname> methods. If the entity does not define a natural id or if the
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natural id it does define is not simple, an exception will be thrown there.
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</para>
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</section>
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<section xml:id="naturalid-mutability-caching">
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<title>Natural Id - Mutability and Caching</title>
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<para>
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A natural id may be mutable or immutable. By default <literal>@NaturalId</literal> marks
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an immutable natural id. An immutable natural id is expected to never change values.
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If the values of the natural id attribute(s) can change, <literal>@NaturalId(mutable=true)</literal>
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should be used instead.
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</para>
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<example>
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<title>Mutable natural id</title>
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<programlisting role="JAVA"><xi:include href="extras/MutableNaturalIdMapping.java" parse="text" /></programlisting>
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</example>
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<para>
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Within the Session, Hibernate maintains a mapping from natural id values to pk values. If natural ids
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values have changed it is possible for this mapping to become out of date until a flush occurs. To work
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around this condition, Hibernate will attempt to discover any such pending changes and adjust for them
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when the <methodname>load</methodname> or <methodname>getReference</methodname> method is executed. To
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be clear: this is only pertinent for mutable natural ids.
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</para>
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<para>
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This "discovery and adjustment" have a performance impact. If an application is certain that none of its
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mutable natural ids already associated with the Session have changed, it can disable that checking by
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calling <methodname>setSynchronizationEnabled(false)</methodname> (the default is true). This will force
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Hibernate to circumvent the checking of mutable natural ids.
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</para>
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<example>
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<title>Mutable natural id synchronization use-case</title>
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<programlisting role="JAVA"><xi:include href="extras/MutableNaturalIdSynchronization.java" parse="text" /></programlisting>
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</example>
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<para>
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Not only can this NaturalId-to-PK resolution be cached in the Session, but we can also have it cached in
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the second-level cache if second level caching is enabled.
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</para>
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<example>
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<title>Natural id caching</title>
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<programlisting role="JAVA"><xi:include href="extras/NaturalIdCaching.java" parse="text" /></programlisting>
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</example>
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</section>
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</chapter>

‎documentation/src/main/docbook/mappingGuide/en-US/chapters/natural_id/extras/MutableNaturalIdMapping.java‎

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@Entity
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public class Person {
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@Id
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private Integer id;
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@NaturalId(mutable=true)
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private String ssn;
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...
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}

‎documentation/src/main/docbook/mappingGuide/en-US/chapters/natural_id/extras/MutableNaturalIdSynchronization.java‎

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Session session = ...;
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Person person = session.bySimpleNaturalId( Person.class )
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.load( "123-45-6789" );
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person.setSsn( "987-65-4321" );
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...
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// returns null!
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person = session.bySimpleNaturalId( Person.class )
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.setSynchronizationEnabled( false )
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.load( "987-65-4321" );
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// returns correctly!
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person = session.bySimpleNaturalId( Person.class )
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.setSynchronizationEnabled( true )
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.load( "987-65-4321" );

‎documentation/src/main/docbook/mappingGuide/en-US/chapters/natural_id/extras/NaturalIdCaching.java‎

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@Entity
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@NaturalIdCache
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public class Company {
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@Id
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private Integer id;
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@NaturalId
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private String taxIdentifier;
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...
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}

‎documentation/src/main/docbook/mappingGuide/en-US/chapters/natural_id/extras/NaturalIdLoadAccessUsage.java‎

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Session session = ...;
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Company company = session.byNaturalId( Company.class )
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.using( "taxIdentifier", "abc-123-xyz" )
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.load();
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PostalCarrier carrier = session.byNaturalId( PostalCarrier.class )
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.using( "postalCode", new PostalCode( ... ) )
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.load();
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Department department = ...;
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Course course = session.byNaturalId( Course.class )
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.using( "department", department )
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.using( "code", "101" )
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.load();

‎documentation/src/main/docbook/mappingGuide/en-US/chapters/natural_id/extras/NonSimpleNaturalIdMapping.java‎

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@Entity
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public class Course {
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@Id
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private Integer id;
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@NaturalId
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@ManyToOne
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private Department department;
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@NaturalId
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private String code;
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...
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}

‎documentation/src/main/docbook/mappingGuide/en-US/chapters/natural_id/extras/SimpleBasicNaturalIdMapping.java‎

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@Entity
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public class Company {
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@Id
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private Integer id;
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@NaturalId
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private String taxIdentifier;
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...
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}

‎documentation/src/main/docbook/mappingGuide/en-US/chapters/natural_id/extras/SimpleCompositeNaturalIdMapping.java‎

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@Entity
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public class PostalCarrier {
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@Id
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private Integer id;
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@NaturalId
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@Embedded
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private PostalCode postalCode;
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...
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}
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@Embeddable
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public class PostalCode {
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...
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}

‎documentation/src/main/docbook/mappingGuide/en-US/chapters/natural_id/extras/SimpleNaturalIdLoadAccessUsage.java‎

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Session session = ...;
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Company company = session.bySimpleNaturalId( Company.class )
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.load( "abc-123-xyz" );
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PostalCarrier carrier = session.bySimpleNaturalId( PostalCarrier.class )
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.load( new PostalCode( ... ) );

‎documentation/status.md‎

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Overall the plan is to define 3 DocBook-based guides. The intention is for this document to serve
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as an outline of the work and a status of what still needs done.
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NOTE : entries marked with <strike>strike-through</strike> indicate that the content is believed to be done; review
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would still be appreciated.
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710

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User Guide
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==========
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Covers reference topics targeting users.
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* Prefix (done)
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* Architecture (done)
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* DomainModel (done)
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* Bootstrap (done)
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* PersistenceContext (done)
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* Database_Access (done)
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* Transactions (done)
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* JNDI (done)
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* <strike>Prefix</strike>
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* <strike>Architecture</strike>
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* <strike>DomainModel</strike>
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* <strike>Bootstrap</strike>
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* <strike>PersistenceContext</strike>
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* <strike>Database_Access</strike>
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* <strike>Transactions</strike>
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* <strike>JNDI</strike>
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* Locking (needs some work)
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* Fetching (needs some work)
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* Batching (needs lot of work - not started - open questions)
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* Native_Queries (needs lots of work)
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* Multi_Tenancy (needs some work)
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* OSGi (right place for this?)
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* Envers
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* Envers (right place for this?)
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* Portability (needs some work)
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matter of pulling that content in and better organizing it.
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* Prefix (done)
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* Data_Categorizations (done)
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* Basic_Types (done)
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* Composition (done)
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* Collection (needs some work)
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* <strike>Prefix</strike>
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* <strike>Data_Categorizations</strike>
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* <strike>Basic_Types</strike>
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* <strike>Composition</strike>
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* <strike>Collection (needs some work)
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* Entity (needs some work)
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* Secondary_Tables (not started)
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* Identifiers (mostly done - needs "derived id" stuff documented)
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* Natural_Id (not started)
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* <strike>Natural_Id</strike>
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* Associations (not started)
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* Attribute_Access (not started)
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* Mapping_Overrides - AttributeOverrides/AssociationOverrides (not started)

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