I have a class called WebAsset:
public class WebAsset {
private Long id;
private String url;
private int status;
//more fields that are not relevent
}
I need to be able to show relationships between WebAsset, so I created a table for the relationship and a composite key class.
public class WebAssetReferencePK {
private Long sourceAssetId;
private Long targetAssetId;
}
public class WebAssetReference {
private WebAssetReferencePK wpk;
private Long updateTime;
}
We are forced to use an older version of Hibernate so we need to use xml files instead of annotaions. Here is the mapping for the reference class:
<class name="ca.gc.cra.www.crawler.valueobject.WebAssetReference" table="webassetreference">
<composite-id name="webAssetReferencePK" class="ca.gc.cra.www.crawler.valueobject.WebAssetReferencePK">
<key-property name="sourceAsset" type="java.lang.Long" column="sourceAssetId" />
<key-property name="targetAsset" type="java.lang.Long" column="targetAssetId" />
</composite-id>
<property name="updateTime" type="java.lang.Long" column="updatetime" not-null="true" />
</class>
In the composite key I get what I expect in the database with 2 ids related to each other. But when I try to query with HQL or Criteria it doesn't work since there is no direct relation between the PK class and WebAsset and I need to be able to do a join between WebAsset and WebAssetReference. If I try to change the composite key types from java.lang.Long to WebAsset then hibernate stores the whole object in the WebAssetReference table instead of just the ids.
An example of what I am trying to do is if I have a sourceAssetId I want to return all the targetAssetIds with the same source, but I don't want the ids themselves I want the WebAsset that is the primary key for each targetAssetId.
I have been searching around for the answer but every example I can find are just simple examples that don't relate.
Update 1: With continued searching I finally found the answer. Instead of key-property I need to use key-many-to-one. I haven't tried a join yet but everything else looks right so this should be the answer.
Update 2: Can't get the query to work with HQL. Here is th SQL of what I am trying to do:
select * from webasset as wa join webassetreference as war on war.targetassetid=wa.webasset_id where war.sourceassetid=?
Here is the HQL that is not working:
FROM WebAsset JOIN WebAssetReference WebAssetReference.WebAssetReferencePK.targetAsset=WebAsset WHERE WebAssetReference.WebAssetReferencePK.sourceAsset = :sourceAsset
I get the following error with HQL: ERROR - line 1:89: unexpected token: .
I'll keep trying but I can't seem to figure out the HQL.
I discovered how to do this. In the case I have above it will not work since I have 2 columns joining to the same table. However if I use the same WebAsset class above and instead use this class:
public class TreeNode implements Comparable<TreeNode>{
private String nodeUrl;
private Long id;
private Boolean folder;
private transient WebAsset nodeAsset = null;
}
With this .hbm.xml file:
<class name="ca.gc.cra.www.crawler.valueobject.TreeNode" table="TreeNode">
<id name="id" type="java.lang.Long" column="treenode_id" >
<generator class="identity"/>
</id>
<many-to-one name="nodeAsset" class="ca.gc.cra.www.crawler.valueobject.WebAsset" column="nodeAsset_id" lazy="false" not-null="false" cascade="none" unique="true" />
<property name="folder" type="java.lang.Boolean" column="folder" not-null="true" />
<property name="nodeUrl" length="512" type="java.lang.String" column="nodeUrl" not-null="true" />
<set name="children" table="TreeNode" inverse="false" lazy="true" >
<key column="parentnode_id"/>
<one-to-many class="ca.gc.cra.www.crawler.valueobject.TreeNode" />
</set>
</class>
You can then use this code to retrieve the join:
Session session = HibernateUtil.getSession();
try {
String hql = "FROM TreeNode tn JOIN tn.nodeAsset WHERE tn.id=5";
Query query = session.createQuery(hql);
List result = query.list();
System.out.println("done");
} catch (HibernateException e) {
e.printStackTrace();
throw new Exception("Query failed", e);
} finally {
session.flush();
session.close();
}
Hibernate can then perform the join correctly. The result will be a List containing an Object array for each entry. The Object contains the 2 classes that are part of the join. You have to cast the Object with (Object[]) to access the elements and then cast each on to the appropriate class.
I would recommend against this approach because Hibernate will attempt to load all connected classes as well. With the example above I was getting 1 row from TreeNode yet it generated 19 select statements. I even attempted to set the connected classes to lazy load and it still generated all the selects.
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