Question

I need to create a relation in Hibernate, linking three tables: Survey, User and Group.
The Survey can be visible to a User or to a Group, and a Group is form of several Users.

My idea was to create a superclass for User and Group, and create a ManyToMany relationship between that superclass and Survey.

My problem is that Group, is not map to a table, but to a view, so I can’t split the fields of Group among several tables -which would happen if I created a common superclass-.

I thought about creating a common interface, but mapping to them is not allowed.
I will probably end up going for a two relations solution (Survey-User and Survey-Group), but I don’t like too much that approach.

I thought as well about creating a table that would look like:

  Survey Id  |  ElementId  | Type

ElementId would be the Group or UserId, and the type… the type of it.
Does anyone know how to achieve it using hibernate annotations? Any other ideas?

Thanks a lot

Answer 1

I posted a very similar answer yesterday. To summarize, you can’t use a mapped superclass because a mapped superclass is not an entity and can’t be part of an association (which is what you want) but you can use an abstract Entity with a TABLE_PER_CLASS inheritance strategy to obtain a similar result.

Something like this (not tested):

@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class AbstractEntity {
    @Id @GeneratedValue(strategy = GenerationType.TABLE)
    private Long id;

    @ManyToMany(mappedBy="entities")
    private Set<Survey> surveys = new HashSet<Survey>();
    ...

}

@Entity
public class User extends AbstractEntity {
    ...
}

@Entity
public class Group extends AbstractEntity {
    ...
}

@Entity
public class Survey {
    @Id @GeneratedValue
    private Long id;

    @ManyToMany
    private Set<AbstractEntity> entities = new HashSet<AbstractEntity>();

    ...
}

References

• Annotations, inheritance and interfaces
• using MappedSuperclass in relation one to many
• Polymorphic association to a MappedSuperclass throws exception