@Entity
@Table (name="DIRECTORY_ITEM")
@Access(AccessType.FIELD)
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="ITEM_TYPE", discriminatorType=DiscriminatorType.STRING, length=100)
public abstract class AbstractItem<T extends AbstractItemData> {

  @Transient
  private transient Class<T> itemDataClass;

  @Id
  private Long id;

  //discriminator field
  @Column(name="ITEM_TYPE", length=100, nullable=false, updatable=false, insertable=false) 
  private String itemType;

  @OneToMany(
      fetch=FetchType.LAZY, 
      mappedBy="item",
      cascade=CascadeType.ALL)
  private Set<T> itemDataSet;

  public AbstractItem(Class<T> itemDataClass) {
    this.itemDataClass = itemDataClass;
  }

    other fields...

    getter and setter....
}

@Entity
@DiscriminatorValue("FOLDER")
public class Folder extends AbstractItem<FolderData> {

  public Folder() {
    super(FolderData.class);
  }
}


@Entity
@DiscriminatorValue("ROOT")
public class Root extends AbstractItem<RootData> {

  public Root() {
    super(RootData.class);
  }
}

@Entity
@Table(name="ITEM_DATA_INDEX")
@Access(AccessType.FIELD)
@Inheritance(strategy=InheritanceType.JOINED)
@Configurable
public abstract class AbstractItemData {

    @Id   
  private Long id;

    @ManyToOne(fetch = FetchType.EAGER)
  @JoinColumn(name = "FK_ITEM_ID", nullable = false, insertable=true, updatable=false)
  private AbstractItem<? extends AbstractItemData> item;

  @Column(name="VALID_FROM", nullable=false, columnDefinition = "TIMESTAMP")
  private Date validFrom;

    @Column(name="VALID_TO", nullable=false, columnDefinition = "TIMESTAMP")
  private Date validTo;

    others fields....
    getters and setters...
}

@Entity
@Table(name="ITEM_FOLDER")
@PrimaryKeyJoinColumn(name="ITEM_DATA_ID")
public class FolderData extends AbstractItemData {

  @Column(name="LABEL", length=400, nullable=false)
  private String label;

    others fields....
    getters and setters...
}

@Entity
@Table(name="ITEM_ROOT")
@PrimaryKeyJoinColumn(name="ITEM_DATA_ID")
public class RootData extends AbstractItemData{

  @Column(name="LABEL1", length=400, nullable=false)
  private String label1;

    others fields....
    getters and setters...
}

SELECT ITEM_DATA_ID, CREATED_BY, CREATED_DATE, LAST_MODIFIED_BY, LAST_MODIFIED_DATE, 
       VALID_FROM, VALID_TO, FK_ITEM_ID 
  FROM ITEM_DATA_INDEX   

SELECT t0.ITEM_DATA_ID, t0.CREATED_BY, t0.CREATED_DATE, t0.LAST_MODIFIED_BY, 
       t0.LAST_MODIFIED_DATE, t0.VALID_FROM, t0.VALID_TO, t0.FK_ITEM_ID, 
       t1.ITEM_DATA_ID, t1.LABEL 
  FROM ITEM_DATA_INDEX t0, ITEM_FOLDER t1 
 WHERE (t1.ITEM_DATA_ID = t0.ITEM_DATA_ID)

SELECT t0.ITEM_DATA_ID, t0.CREATED_BY, t0.CREATED_DATE, t0.LAST_MODIFIED_BY, 
       t0.LAST_MODIFIED_DATE, t0.VALID_FROM, t0.VALID_TO, t0.FK_ITEM_ID, 
       t1.ITEM_DATA_ID, t1.LABEL1 
  FROM ITEM_DATA_INDEX t0, ITEM_ROOT t1 
 WHERE (t1.ITEM_DATA_ID = t0.ITEM_DATA_ID)

      // Search folder with code '|132497614638538'");
      Folder folder = new Folder("|132497614638538");
      folder = repository.load(folder);
      
      // generated sql:
      //      SELECT ITEM_ID, ITEM_TYPE, CREATED_BY, CREATED_DATE, ITEM_CODE,
      //             LAST_MODIFIED_BY, LAST_MODIFIED_DATE 
      //        FROM DIRECTORY_ITEM 
      //        WHERE ((ITEM_TYPE = ?) AND (ITEM_CODE = ?))
      // bind => [FOLDER, |132497614638538]

      // the Folder class is loaded correctly


      if(folder != null) {
        Set<FolderData> folderDataSet = folder.getAllData();  // get lazy data
        
        // generated sql:
        // SELECT t0.ITEM_DATA_ID, t0.CREATED_BY, t0.CREATED_DATE, t0.LAST_MODIFIED_BY,
        //        t0.LAST_MODIFIED_DATE, t0.VALID_FROM, t0.VALID_TO, t0.FK_ITEM_ID,
        //        t1.ITEM_DATA_ID, t1.LABEL1 
        //   FROM ITEM_DATA_INDEX t0, ITEM_ROOT t1 
        //  WHERE ((t0.FK_ITEM_ID = ?) AND (t1.ITEM_DATA_ID = t0.ITEM_DATA_ID))
	// bind => [170]

        // the join should be between ITEM_DATA_INDEX and ITEM_FOLDER and not between 
        // ITEM_DATA_INDEX and ITEM_ROOT

        System.out.println("folderData found: " + folderDataSet.size()); // output: 0
        
      }