Conditional/composite JoinColumn In JPA/Hibernate

Question

In one of my classes, say Location, I have something like this:

private List<Magician> magicians;

...

@OneToMany(fetch=FetchType.LAZY, cascade = CascadeType.ALL)
@JoinColumn(name="location_id")
public List<Magician> getMagicians() {
    return magicians;
} 

public void setMagicians(List<Magician> magicians) {
    this.magicians = magicians;
}

where Magician has variables

private Integer id;
private Integer location_id;
private Boolean active;

Now I would like to modify the getter annotation so as to get only the magicians for which active is true.

How can I achieve this?

Thanks for your attention.

Answer 1

In Hibernate my issue can be solved using a filter, as explained here : https://stackoverflow.com/a/6920847/1966869. The xml version, which I have just tested and appears quite practical, is described here : http://www.mkyong.com/hibernate/hibernate-data-filter-example-xml-and-annotation/. There might be other solutions using @Where (How to use @Where in Hibernate) or @JoinFormula (@JoinFormula and @OneToMany definition - poor documentation), but I have not tested them.

Answer 2

First of all, Magician should not have a location_id field. It should have a field of type Location. This would make a bidirectional association between the two entities:

@OneToMany(fetch=FetchType.LAZY, cascade = CascadeType.ALL, mappedBy = "location")
public List<Magician> getMagicians() {
    return magicians;
} 

...

@ManyToOne
@JoinColumn(name = "location_id")
public Location getLocation() {
    return this.location;
}

Then, to answer your question, the state and associations of an entity are not used to implement a speciic use-case or query. Thy're used to model what the database contains. To get the active magicians of a given location, you should simply use a query:

select m from Magician m where m.active = true and m.location = :location

Answer 3

I wouldn't perform this work at the entity level, instead I would perform it using the EntityManager to create a query. As JB mentions you need to add location as a composite object instead of an Integer.

//In Method

List<Magician> magicians = 
   em.createQuery("select m from Location l join Magician m where m.active = true and
   l.[specify id here] = :locationId", Magician.class).setParameter("locationId",
   1).getResultList();