同一类型实体的多对多关系

Question

我有一个实体如下。我很好奇是否可以建立关系，正如我将在示例中描述的那样：

我正在创建 2 个 Person 实体 Michael 和 Julia.
我正在将 Julia 添加到 Michael 的好友集中。

之后，我将检索 Michael 作为 JSON 响应，Julia 在响应中可用。但是当我检索 Julia 时，她的朋友集是空的。我想通过只保存友谊的一侧来建立双向友谊关系。我想在不做任何其他操作的情况下让 Michael 进入 Julia 的朋友组。我认为它必须由 Hibernate 管理。这可能吗？我应该怎么做？

@ToString(exclude = "friends") // EDIT: these 2 exclusion necessary
@EqualsAndHashCode(exclude = "friends")
public class Person{

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "id")
private Long id;

@Column(name = "name",unique = true)
private String name;

@JsonIgnoreProperties("friends") // EDIT: will prevent the infinite recursion
@ManyToMany(cascade = CascadeType.ALL)
@JoinTable(name = "FRIENDSHIP",
       joinColumns = @JoinColumn(name = "person_id", 
referencedColumnName = "id"),
       inverseJoinColumns = @JoinColumn(name = "friend_id", 
referencedColumnName = "id"))
private Set<Person> friends;

下面是我创建友谊的服务层代码：

    @Override
    public Person addFriend(String personName, String friendName) 
        throws FriendshipExistsException, PersonNotFoundException {
    Person person = retrieveWithName(personName);
    Person friend = retrieveWithName(friendName);
    if(!person.getFriends().contains(friend)){
        person.getFriends().add(friend);
        return repository.save(person);
    }
    else{
        throw new FriendshipExistsException(personName, friendName);
    }
}

Related Question:

Updated the source code and this version is working properly.

Answer 1

// Creating a graph to help hibernate to create a query with outer join.
@NamedEntityGraph(name="graph.Person.friends",
    attributeNodes = @NamedAttributeNode(value = "friends"))
class Person {}

interface PersonRepository extends JpaRepository<Person, Long> {
    // using the named graph, it will fetch all friends in same query
    @Override
    @EntityGraph(value="graph.Person.friends")
    Person findOne(Long id);
}

@Override
public Person addFriend(String personName, String friendName) 
    throws FriendshipExistsException, PersonNotFoundException {
    Person person = retrieveWithName(personName);
    Person friend = retrieveWithName(friendName);
    if(!person.getFriends().contains(friend)){
        person.getFriends().add(friend);
        friend.getFriends().add(person); // need to setup the relation
        return repository.save(person); // only one save method is used, it saves friends with cascade
    } else {
        throw new FriendshipExistsException(personName, friendName);
    }

}

如果您检查休眠日志，您将看到：
Hibernate：插入 person (name, id) 值 (?, ?)
Hibernate：插入 person (name, id) 值 (?, ?)
休眠：插入友谊（person_id，friend_id）值（？，？）
休眠：插入友谊（person_id，friend_id）值（？，？）

同一类型实体的多对多关系

Many to many relationship for same type entity

java

spring

many-to-many

hibernate

bidirectional-relation