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Update Projection section in reference documentation. #3449

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Update Projection section in reference documentation. #3449

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Prepare issue branch.
christophstrobl Apr 26, 2024
9a16062
Update Projection section in reference documentation.
christophstrobl Apr 26, 2024
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2 changes: 1 addition & 1 deletion pom.xml
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Expand Up @@ -5,7 +5,7 @@

<groupId>org.springframework.data</groupId>
<artifactId>spring-data-jpa-parent</artifactId>
<version>3.3.0-SNAPSHOT</version>
<version>3.3.x-3286-SNAPSHOT</version>
<packaging>pom</packaging>

<name>Spring Data JPA Parent</name>
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4 changes: 2 additions & 2 deletions spring-data-envers/pom.xml
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Expand Up @@ -5,12 +5,12 @@

<groupId>org.springframework.data</groupId>
<artifactId>spring-data-envers</artifactId>
<version>3.3.0-SNAPSHOT</version>
<version>3.3.x-3286-SNAPSHOT</version>

<parent>
<groupId>org.springframework.data</groupId>
<artifactId>spring-data-jpa-parent</artifactId>
<version>3.3.0-SNAPSHOT</version>
<version>3.3.x-3286-SNAPSHOT</version>
<relativePath>../pom.xml</relativePath>
</parent>

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2 changes: 1 addition & 1 deletion spring-data-jpa-distribution/pom.xml
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Expand Up @@ -14,7 +14,7 @@
<parent>
<groupId>org.springframework.data</groupId>
<artifactId>spring-data-jpa-parent</artifactId>
<version>3.3.0-SNAPSHOT</version>
<version>3.3.x-3286-SNAPSHOT</version>
<relativePath>../pom.xml</relativePath>
</parent>

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4 changes: 2 additions & 2 deletions spring-data-jpa/pom.xml
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Expand Up @@ -6,7 +6,7 @@

<groupId>org.springframework.data</groupId>
<artifactId>spring-data-jpa</artifactId>
<version>3.3.0-SNAPSHOT</version>
<version>3.3.x-3286-SNAPSHOT</version>

<name>Spring Data JPA</name>
<description>Spring Data module for JPA repositories.</description>
Expand All @@ -15,7 +15,7 @@
<parent>
<groupId>org.springframework.data</groupId>
<artifactId>spring-data-jpa-parent</artifactId>
<version>3.3.0-SNAPSHOT</version>
<version>3.3.x-3286-SNAPSHOT</version>
<relativePath>../pom.xml</relativePath>
</parent>

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25 changes: 25 additions & 0 deletions src/main/antora/modules/ROOT/pages/repositories/projections.adoc
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include::{commons}@data-commons::page$repositories/projections.adoc[leveloffset=+1]

NOTE: It is important to note that <<projections.dtos,Class-based projections>> with JPQL is limited to *constructor expressions* in your JPQL expression, e.g. `SELECT new com.example.NamesOnly(u.firstname, u.lastname) from User u`. (Note the usage of a FQDN for the DTO type!) This JPQL expression can be used in `@Query` annotations as well where you define any named queries. And it's important to point out that class-based projections do not work with native queries AT ALL. As a workaround you may use named queries with `ResultSetMapping` or the Hibernate specific https://docs.jboss.org/hibernate/orm/6.0/javadocs/org/hibernate/transform/ResultTransformer.html[`ResultTransformer`]

[NOTE]
====
Some JPA providers may require additional hints when working with <<projections.dtos,Class-based projections>> especially when using those directly with method derived queries.
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That confuses me. @PersistenceCreator is a Spring Data annotation, I'm sure that JPA providers don't care about that.

I would expect that the situation is more: We need either a constructor taking all the properties as arguments and if it isn't the only constructor we need it to be marked with @PersistenceCreator.

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It's our code that determines input properties. We use our mechanism to discover whether there is a preferred constructor and the properties it takes.

If you are facing errors like `JpaSystemException: Specified result type did not match Query selection type` make sure to provide a constructor hint via `@PersistenceCreator` to be passed on, as outlined below:
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It isn't so much outlined below but demonstrated, isn't it?


[source,java]
----
public class NamesOnly {

private final String firstname;
private final String lastname;

protected NamesOnly() { }

@PersistenceCreator
public NamesOnly(String firstname, String lastname) {
this.firstname = firstname;
this.lastname = lastname;
}

// ...
}
----
====
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