MapKey problems

Zerg-Spirit · **Joined:** Wed Jun 13, 2007 5:44 am **Posts:** 16

Hi!

I'm trying to map a Collection correctly, but somehow can't do it.

Here's my main entity.
I have a collection here constructed as a map. I'd like the map key to be a String, the stringId property of Directory, which is a property of Entry.
But it doesn't work

Code:

@Entity
public class EntryOwner {

   @OneToMany(mappedBy = "entryOwner")
   @MapKey(name = "directory.stringId")
   private Map<String, Entry> entries = new HashMap<String, Entry>();
}

Code:

@Entity
public class Entry {
   @NotNull
   @ManyToOne
   private Directory directory;
   @OneToOne
   private EntryOwner entryOwner;
}

Code:

@Entity
public class Directory implements Serializable {
   @OneToMany(cascade = CascadeType.ALL, mappedBy = "directory")
   private List<Entry> entries;
   private String stringId;
}

But, as stated at the beginning of my post, it doesn't work, as I got this error:

Code:

Map key property not found: com.lrb.modules.directory.Entry.directory.stringId

How to map that correctly?

Froestel · **Posted:** Wed Sep 16, 2009 12:35 pm

Hi Zerg-Spirit,

Hibernate is looking for the specified map key in Entry and does not find it. There is no field/column of that name.

Create a back reference in Entry to Directory.stringID and use that for the map key. That might help.

CU
Froestel

Zerg-Spirit · **Joined:** Wed Jun 13, 2007 5:44 am **Posts:** 16

Thank you.

How would you do that back reference. It would need to be a property Hibernate would fetch from the directory automatically, not a useless field in the database.

Froestel · **Posted:** Thu Sep 17, 2009 6:43 am

Hi Zerg-Spirit,

in http://docs.jboss.org/hibernate/stable/annotations/reference/en/html/entity.html#entity-mapping-association-collections the Hibernate Annotations docs say

Quote:

EJB3 allows you to map Maps using as a key one of the target entity property using @MapKey(name="myProperty") (myProperty is a property name in the target entity). When using @MapKey (without property name), the target entity primary key is used. The map key uses the same column as the property pointed out: there is no additional column defined to hold the map key, and it does make sense since the map key actually represent a target property.

Therefore, you will need a column in the same table (a.k.a. a field in the same entity) for Hibernate to refer to.

I guess the fastest - but actually not very nice - way to do it, is having a field in Entity that receives the stringID value as soon as Entity's Directory is set. That field should be mapped to the Entity table. This is not very nice because DB integrity cannot be guaranteed as long as you do not "connect" the stringID of Entity and that of Directory on DB level with a constraint. It also produces redundant data. You don't want that.

See, i.e., http://docs.jboss.org/hibernate/stable/annotations/reference/en/html/entity.html#d0e2273 from the Hibernate Annotations documentation about the @Formula annotation. This can probably help you out by creating a virtual column to refer to. I don't have so much experience with annotations but when using mapping files the <map-key> element has an optional attribute formula that is probably exactly made for this purpose.

CU
Froestel