EJB QL/ JPQL and search for inheritance

[jrosskopf]

Hello,

I have a problem with creating a QL expression for some of my entities. First of all I have an entity "Item" which has a ManyToMany reference to 0..* "Tags" But Tag is just a base class for more complex Entities such as "PlaceMark"

@Entity
public class Item implements Serializable{
 private static final long serialVersionUID = -6240744395949853067L;
 .
 .
 .
 /**
 * Get a list of tags associated with the item
 * @return list of tags associated with the item
 */
 @ManyToMany(cascade={CascadeType.PERSIST, CascadeType.MERGE})
 public List<Tag> getTags() {
 return tags;
 }
 public void setTags(List<Tag> tags) {
 this.tags = tags;
 }
}

@Entity
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="dtype", discriminatorType=DiscriminatorType.STRING)
@DiscriminatorValue("Tag")
public class Tag implements Serializable{
 private static final long serialVersionUID = 2432906233376577675L;

 long gid;
 String name;
 List<Item> items;

 @Id
 @GeneratedValue
 public long getGid() {
 return gid;
 }
 public void setGid(long gid) {
 this.gid = gid;
 }

 public String getName() {
 return name;
 }
 public void setName(String value) {
 this.name = value;
 }

 @ManyToMany(mappedBy="tags", cascade={CascadeType.PERSIST, CascadeType.MERGE})
 public List<Item> getItems() {
 return items;
 }
 public void setItems(List<Item> items) {
 this.items = items;
 }
}

@Entity
@DiscriminatorValue("PlaceMark")
public class PlaceMark extends Tag implements Serializable {
 private static final long serialVersionUID = -6646702308712255267L;

 boolean visibibilty;
 LookAt lookAt;
 Point point;

 public PlaceMark() {
 super.setName("PlaceMark");
 }

 @Embedded
 public LookAt getLookAt() {
 return lookAt;
 }
 public void setLookAt(LookAt lookAt) {
 this.lookAt = lookAt;
 }

 @Embedded()
 public Point getPoint() {
 return point;
 }
 public void setPoint(Point point) {
 this.point = point;
 }

 public boolean isVisibibilty() {
 return visibibilty;
 }
 public void setVisibibilty(boolean visibibilty) {
 this.visibibilty = visibibilty;
 }

 public void setName(String value) {
 }

}

So now my question, how can I search for items annotaded with different types of tags in the "Tag" class hierachy?

Thank you in advance.
Regards
---
Joachim

[andydale]

Hi Joachim,

In a EJB3 QL query you can query the Discriminator Value in the where section of the query. For example:

WHERE tag.class = <discriminator value>

This is how i have managed to restrict results based on a Discriminator Value before.


Cheers,

Andy

[jrosskopf]

Hi Andy (and everybody else),

thank you for fast reply.

That works well and is a first step torwards a solution.
But I still need access to properties of the subclasses in the collection.

Speaking in plain java it would be condition like this:

if (tag instanceof PlaceMark && ((PlaceMark)tag).getPoint().getX() == 0.0d)

So is it possible to do something like a cast? I read through Mr Monsons-Haefels and Mr Brukes book an found nothing useful.

Regards
---
Joachim

[andydale]

Joachim,

I have never attmepted to do an instanceof with the inheritance type of a single table per hierarchy. Prehaps you should investigate some other strategies, namely table per subclass as i think this will provide you with the functionality you would like, like so:

 //create query
 Query tmpQuery = mEntityManager.createQuery("FROM Tag t");

 //get query result
 ArrayList<Tag> queryResult = (ArrayList<Tag>)tmpQuery.getResultList();

 //get iterator
 Iterator it = testList.iterator();

 //loop
 while (it.hasNext()) {
 Tag element = (Tag) it.next();

 if(element instanceof PlaceMark) {
 //DO STUFF :-)
 }
 }

In your top level parent class you just need to annotate like so:

@Inheritance(strategy = InheritanceType.JOINED)

and i just have a getter and setter for the id in this parent class, and not in the sub-classes(children). All you should need to do in the children classes is just extend the parent class.

To persist, just create and persist a child object and it automatically create and persists the parent objet with the correct attributes.


Cheers,

Andy

[joe.marques]

"jrosskopf" wrote:


Speaking in plain java it would be condition like this:

if (tag instanceof PlaceMark && ((PlaceMark)tag).getPoint().getX() == 0.0d)

---
Joachim

Joachim, did you ever find a way to do this at the JPQL level?