Question

I have the following @Entities

@Entity
public class Configuration{
   @OneToMany
   protected Map<String, Component> components;
}

and

@Entity
public class Component{
    protected String displayName;
}      

I do not understand why this works, returning all Configurations

  CriteriaBuilder cb = em.getCriteriaBuilder();
  CriteriaQuery<Configuration> cq = cb.createQuery(Configuration.class);
  Root<Configuration> pc = cq.from(Configuration.class);
  cq.select(pc);

But if I do a MapJoin, even without setting any conditions, it does not return anything

  CriteriaBuilder cb = em.getCriteriaBuilder();
  CriteriaQuery<Configuration> cq = cb.createQuery(Configuration.class);
  Root<Configuration> pc = cq.from(Configuration.class);
  MapJoin<Configuration, String, Component> mapJoin = pc.join(Configuration_.components); 
 cq.select(pc);

What am I missing? I’m at a loss, I’ve been through the tutorials, but have not found the answers I need. Any help much appreciated.

Answer 1

Because the join type is inner by default, which means that for a configuration to be returned it has to at least have one component. If none of your configurations have a component, nothing is returned.

The first query is equivalent to

select configuration.* from configuration

And the second one is equivalent to

select configuration.* from configuration
inner join component on component.id = configuration.id