Translate SQL Left Join query to EclipseLink JPA query

Question

I am trying to write a JPA NamedQuery. I have a working SQL query as follows:

SELECT MIN(t1.Id + 1) AS nextID 
FROM 
    MyTable t1 LEFT JOIN MyTable t2 
ON 
    t1.Id + 1 = t2.Id
WHERE 
    t2.Id IS NULL

I am not able to translate this query to JPQL syntax.

Answer 1

I think doing such a custom join is not possible with standard JPQL. I was looking for a possibility to do it some time ago and found that Hibernate offers a proprietary extension @JoinFormula to achieve this, cf. Hibernate @JoinFormula . But I couldn't find an equivalent for EclipseLink.

You might be able use a @NamedNativeQuery together with an @SqlResultSetMapping to map your SQL statement to a JPA entity, something like this:

@NamedNativeQuery( name = "customJoin", 
                   query = "SELECT MIN(t1.Id + 1) AS nextID FROM MyTable t1 LEFT JOIN MyTable t2 ON t1.Id + 1 = t2.Id WHERE t2.Id IS NULL",
                   resultSetMapping = "customJoinMapping" )

@SqlResultSetMapping( name = "customJoinMapping",
                      entities = @EntityResult( entityClass = MyTable.class, fields = @FieldResult( name = "id", column = "nextID" ) ) )
@Entity
@Access( AccessType.Field )
public class MyTable {
   private int id;

   public int getId() {
      return this.id;
   }

   public void setId( int id ) {
      this.id = id;
   }
}

UPDATE

Much later, I found a way to customize relationship joins in EclipseLink here . The feature to use is a DescriptorCustomizer , which can be placed on a JPA entity with @Customizer .

In the customize() method, one can express additional join criteria using the EclipseLink Expression API. For example, the following code fragment would produce the additional join criteria [...] AND TargetEntity.someAttribute IN ('V', 'W', 'U') (where someRelationship points to a TargetEntity ).

OneToManyMapping mapping = (OneToManyMapping) descriptor.getMappingForAttributeName( "someRelationship" );
Expression origExp = mapping.buildSelectionCriteria();
ExpressionBuilder expBuilder = origExp.getBuilder();
Expression constantExp = expBuilder.get( "someAttribute" ).in( new String[] { "V", "W", "U" } );
Expression newExp = origExp.and( constantExp );
mapping.setSelectionCriteria( newExp );