JPA 具有相同数据库和额外属性的多对多关系

Question

I need a Many-To-Many Relationship within the same Database. I don't mind creating mapping databases, but I want to have only one Entity in the end.

Let's say I've got a resource which can have many resources (Sub-Resources). What I need is an Resource with the Sub-Resources and also the count of them because one Resource can have x resources.

Essentially, I need this with the extra Attribute of the count of Sub resources needed for the Resource.

@Table(name = "resources")
public class Resources {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @Column
    private String name;

    @ManyToMany
    private Collection<Resources> subResources;
    
} 

To clarify that a bit, at best I would have something like that:

@Table(name = "resources")
public class Resources {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @Column
    private String name;

    @ManyToMany
    private HashMap<Resources, Integer /* count */> subResources;
    
} 

I know how it works with two tables (Resources & Sub Resources) and a mapping type, but I couldn't figure out how to do it as described above, since Resources can be Sub-Resources at the same time.

Thanks in advance

EDIT: I need an extra Attribute in the mapping table where I can set the amount of sub resources as an Integer

Answer 1

The configuration yo have will work for a unidirectional relationship. There is no technical problem, only you will not be able to specify the multiple parents of a subresource, so in the end it is not many to many.

To make it trully many to many you need another field on the Resources class to define the inverse side of the relationship; I have added the @JoinTable annotation to make the names in the join table explicit, but it is optional if the defaults are good enough for you; I also switched from the vary basic Collection to List ; I would prefer Set and you would have to provide equals and hashCode on the entity. Finally I am always initializing the collection-valued fields ( ArrayList here; HashSet if you go for Set ), so as to avoid silly NullPointerException s or complex initialization code:

    @ManyToMany
    @JoinTable(
            name = "RESOURCE_SUBRESOURCE",
            joinColumns = @JoinColumn(name = "resource_id"),
            inverseJoinColumns = @JoinColumn(name = "subresource_id")
    )
    private List<Resource> subResources = new ArrayList<>();

    // the mappedBy signals that this is the inverse side of the relation, not a new relation altogether
    @ManyToMany(mappedBy = "subResources")
    private List<Resource> parentResources = new ArrayList<>();

Use as:

    Resources r1 = new Resources();
    r1.setName("alpha");
    em.persist(r1);

    Resources r2 = new Resources();
    r2.setName("beta");
    r2.getSubResources().add(r1);
    em.persist(r2);

    Resources r3 = new Resources();
    r3.setName("gama");
    em.persist(r3);

    Resources r4 = new Resources();
    r4.setName("delta");
    // won't work, you need to set the owning side of the relationship, not the inverse:
    r4.setParentResources(Arrays.asList(r2, r3));
    // will work like this:
    r2.getSubResources().add(r4);
    r3.getSubResources().add(r4);
    // I believe that the order of the following operations is important, unless you set cascade on the relationship
    em.persist(r4);
    r2 = em.merge(r2);
    r3 = em.merge(r3);

As for the count: In the question you mention that you want a count of related objects. While specific JPA providers (Hibernate, EclipseLink) may allow you to accomplish this (using a read-only field that is populated by an aggragate query - COUNT(*) FROM JoinTable WHERE resource_id=? ), it is not standard. You can always do resource.getSubResources().size() , but that would fetch all the subresources into memory, which is not a good thing and might in fact be a really bad thing if you call in frequently or there are many sub/parent resources.

I would prefer to run a separate count query, perhaps even for a set of resource ids, whenever I really need this.