Hibernate Spring OneToMany - ManyToOne领域驱动设计

Question

I have 2 Entities "User" and "Group". Actually this is a ManyToMany association but I want to persist additional informations in the relation table like created or userid.

So I found out that I have to introduce a new entity which represents the relation table "UserGroup".

The tables:

user id, name, pw

group id, name

usergroup id, userid, groupid, created, createduserid

The entities:

@Entity
@Table(name="user")
public class User {
  @OneToMany(mappedBy="user")
  List<UserGroup> groups;
}

@Entity
@Table(name="group")
public class Group {
  @OneToMany(mappedBy="group")
  List<UserGroup> users;
}

@Entity
@Table(name="usergroup")
public class UserGroup {
  @ManyToOne
  User user;

  @ManyToOne
  User group;

  @Column(name="created")
  Date created;

  // ...
}

Now due to Domain Driven Design I would have to introduce in my User Entity methods like: User::addGroup(group), User::removeFromGroup(group). But technically this appears to be impossible because I need to create a UserGroup entity, inject the User and Group object and finally persist it with the EntityManager.

Is there a more elegant solution or do I really have to do this operations outside of my entities in service objects?

(With ManyToMany I can do this in the entities.)

Answer 1

You started your domain description with "entity tables" and "relations between tables" and that is not in any way a DDD approach. You must dedcribe us your domain and ignore the persistence as if all the objects are stored in memory.

Also, you need to use, in code, terms from your ubiquitous language, for example replace "UserGroup" with "Membership".

You should not reference an aggregate root ( Group ) from another aggregate root ( User ). I wouldn't do that even if is a temporary reference as it moves unnecessary knowledge to the referencer about the referenced. You can only reference by ID . So your code should look like this User::addGroupMembership(groupId) and internally create a new instance of Membership .

Is there a more elegant solution or do I really have to do this operations outside of my entities in service objects?

If you make this association from a domain a service you will have an anemic domain model .

Answer 2

Let me point you to the excellent example of Vaughn Vernon when he designed such a relation between User and Group:

https://github.com/VaughnVernon/IDDD_Samples/tree/master/iddd_identityaccess/src/main/java/com/saasovation/identityaccess/domain/model/identity

As mentionned before, if you model the objects with relations between tables, you are doing it wrong. See the concept Vaughn used in his sample to understand how he modeled the fact a user belong to a group.