Question

Following class structure is given:

class Job  {     String description;     Collection<JobHistory> history; }  class JobHistory {     Date assignDate;     User jobOwner; }  class JobOwner  {     String name;     String id; } 

This class-structure is accessible on the db via JPA. In the DAO-Layer I can write queries in JPA syntax.

The Problem: I want a list with Job and JobHistory entries for a given owner with given id and who is the last one in the Jobhistory of the job (ordered by assignDate). Sounds quite complicated, perhaps simpler: give me all jobs and JobHistory where specified owner is the actual owner of the job.

Update: for clarity I will slightly change the names of the classes.

class Job  {     String description;     Collection<JobOwnerHistory> history; }  class JobOwnerHistory {     Date assignDate;     User jobOwner; }  class JobOwner  {     String name;     String id; } 

Every Job has a history of his owners sorted by assignDate. The actual owner got the job last assigned (i.e. MAX(assignDate)). I want find for every job the JobOwnerHistory entry with MAX(assignDate) for a specific user User.

Answer 1

Try:

SELECT j, j.history FROM Job j JOIN User u WHERE u.name = :name 

If I were to do this in EclipseLink, I would change it slightly:

public List<Job> getAllJobsForUser(String username) {   List<Job> jobs = entityManager     .createQuery('SELECT j FROM Job j JOIN User u WHERE u.name = :name')     .setParameter('name', username)     .setHint(QueryHints.BATCH, 'j.history')     .queryForList(); } 

The difference? In the first version, you’re returning two objects, so you have to retrieve them from a List or Object arrays whereas in the second, the query hint just loads all the job histories from an (assumedly) lazyy one-to-many relationship.

I don’t know if Hibernate has an equivalent to this. Toplink Essentials doesn’t. But it’s one of my favourite features of EclipseLink.

Oh and obviously you can (and probably should) use a named query instead of an adhoc query like I’ve done (since those can be verified during the build).