玩2框架多对多关系更好的设计

Question

In my app I have different User s and Item s, so each user can pick many items.

In the tutorial I have learned about @ManyToMany annotation.

@Entity
public class Item extends Model {

    ...

    @ManyToMany(cascade = CascadeType.REMOVE)
    public List<User> users = new ArrayList<User>();

But second option I can think of is to define a separate class for User-to-Item relation so I can add additional information like date and time.

@Entity
public class ItemUserRel extends Model {
    @Id
    public Long id;

    public User user;
    public Item item;

    //additional information
    public Date date;

    ...

Which of both options is better design and why?

Answer 1

I faced a similar issue a while ago. I also had to deal with a model User and the model Group . My requirements were:

A user can have n readable and n writable Groups. These permissions must be stored in a third table (not in User and not in Group table). But also additional properties like authorisedBy and 'authorisedOn'. So @ManyToMany did not worked for, because I had no real control of it. Also the additional properties makes it hard to map via JPA.

Perhaps other designs are possible but I (still) think that introducing a new class UserGroup would be best. This class has @ManyToOne relation to a single User .

I end up defining these three models:

1. User
2. Group - General information about the group model
3. UserGroup - Containing additional fields like: permissions , authorisedBy , authorisedOn etc.

On my User model, I would have getter getUserGroups() but also getPersonalGroup() which is basically one (personal) instance of Group in getUserGroups() but where the createdBy and authorisedBy is the same user.

I found this design much more maintainable by me and more clear. Also this design helped me to create a comfortable user interface, where the administrator can manage and change permissions for UserGroups.

Perhaps more useful information

• Mapping many-to-many association table with extra column(s)
• How Do I Create Many to Many Hibernate Mapping for Additional Property from the Join Table?