Using HQL to select from joined tables having one to many relationship between them

Question

I am stuck with this Hibernate thing and dont know how to figure it out. Please help !

So I have these two tables:

POSITION
positionid(PK), description

JOB
jobid(PK),positionid(FK),description

How do I use HQL in Hibernate to fetch all jobs with their corresponding position descriptions ?

Edit: So this what I am trying to achieve:

JOBID     POSITION.DESCRPTION     JOB.DESCRIPTION
  1        Teacher                Science Teacher
  2        Coach                  Football Coach

and so on for all job's in JOB table. I am trying to figure out what will be HQL for this.

I have put together the following code till now :

position.hbm.xml

<hibernate-mapping>
  <class name="com.XXXX.model.Position" table="POSITION">
   <id name="positionID" type="int" column="POSITIONID" >
   <generator class="assigned"/>
  </id>

  <property name="description">
   <column name="DESCRIPTION" />
  </property>

  <set name="jobs">
    <key column="positionID" />
    <one-to-many class="com.XXXX.model.Job" />
  </set>

 </class>
</hibernate-mapping>

job.hbm.xml

<hibernate-mapping>

  <class name="com.XXXX.model.Job" table="JOB">
   <id name="jobID" type="int" column="JOBID" >
   <generator class="assigned"/>
  </id>

  <property name="description">
   <column name="DESCRIPTION" />
  </property>

  <many-to-one name="position" class="com.XXXX.model.Position" column="positionID" />

 </class>
</hibernate-mapping>

Position.java

public class Position {

    private int positionID;
    private String description;
    private Set<Job> jobs = new HashSet<Job>();

    // Getters and Setters for all the above three follows...

}

Job.java

public class Job {

    private int jobID;
    private String description;
    private Position position;

    // Getters and Setters for all the above three follows...
}

In my code now I use

session.createQuery("from Position as p left join p.positionID as pid").list();

I know its not exactly correct and I am getting the follow error:

java.lang.NullPointerException
    at org.hibernate.hql.ast.HqlSqlWalker.createFromJoinElement(HqlSqlWalker.java:317)
    at org.hibernate.hql.antlr.HqlSqlBaseWalker.joinElement(HqlSqlBaseWalker.java:3268)
    at org.hibernate.hql.antlr.HqlSqlBaseWalker.fromElement(HqlSqlBaseWalker.java:3060)
    at org.hibernate.hql.antlr.HqlSqlBaseWalker.fromElementList(HqlSqlBaseWalker.java:2938)
    at org.hibernate.hql.antlr.HqlSqlBaseWalker.fromClause(HqlSqlBaseWalker.java:688)
    at org.hibernate.hql.antlr.HqlSqlBaseWalker.query(HqlSqlBaseWalker.java:544)
    at org.hibernate.hql.antlr.HqlSqlBaseWalker.selectStatement(HqlSqlBaseWalker.java:281)
    .........................
    ...........................

Can someone tell me how to fix this please ?

Answer 1

If you only want these three columns, then the HQL should be

select job.id, position.decription, job.description 
from Job job
left join job.position position

This query will return a List<Object[]> and each Object array in the list will contain these three elements.

It would be much more natural however to use this query:

select job from Job job
left join fetch job.position

which would load all the jobs with their position. The query would return a List, and you would be able to access the three information uou want using job.getId(), job.getPosition().getDescription(), and job.getDescription().

The syntax of HQL is described with examples in the reference documentation, a must-read.