JPA query for getting the whole tree

Question

I have a class which models all categories and they can be ordered hierarchically.

@Entity
@Table(name=\"categories\")
public class Category {
    @Id
    @G         


        

Answer 1

First of all, assuming in this hierarchy a "father" can have more than one child, then the father field should be annotated as @ManyToOne.

If you have a field that all the members of a tree share, or if the tree contains the entire table, then it is possible to do it with JPA in an efficient way, though not through a single JPA query.

You simply need to prefetch all the members of the tree, and then traverse the tree:

@Entity
@Table(name="categories")
public class Category {
    @Id
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="sequence")
    @SequenceGenerator(name="sequence", sequenceName="categories_pk_seq", allocationSize=1)
    @Column(name="id")
    private Long id;

    @Column
    private String name;

    @ManyToOne
    @JoinColumn(name="idfather")
    private Category father;

    @OneToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE}, 
               fetch = FetchType.LAZY, 
               mappedBy = "idfather")
    @OrderBy("name")
    private List subCategories;
}

Notice the @OrderedBy annotation on the subCategories field.

Now you can get the entire tree by first loading all the categories into a jumbled list, just so that they'd all be in memory, and then traverse the tree.

public List getTree() {
    List jumbled = 
        entityManager.createQuery("from Category", Category.class).getResultList();

    Category root = null;
    for(Category category : jumbled) {
        if(category.getFather() == null) {
            root = category;
            break;
        }
    }

    List ordered = new ArratList();
    ordered.add(root);
    getTreeInner(root, ordered);
}

private void getTreeInner(Category father, List ordered) {
    for(Category child : father.getSubCategories()) {
        ordered.add(child);
        getTreeInner(child, ordered);
    }
}

I'm only learning JPA myself right now, so I may be missing something crucial, but this approach seems to work for me.