如何在不删除实际目标实体的情况下删除 @ManyToMany 关联（加入链接）

Question

我正在开发一个 Spring-MVC 应用程序，其中包含多对多映射。有3张表，一张是GroupAccount、memberjunction、GroupMembers。现在，当我删除 groupMember 时，我只想从 GroupMembers 和与 memberId 对应的 memberjunction 中删除条目。我尝试使用 Cascade.Remove，它也导致 GroupAccount 中的删除。我正在发布我的 sql 代码，以及我到目前为止所拥有的。

SQL 代码：

CREATE TABLE groupaccount
(
  groupid numeric NOT NULL,
  groupname character varying,
  adminusername character varying,
  CONSTRAINT groupid PRIMARY KEY (groupid)
)
CREATE TABLE memberjunction
(
  memberid integer NOT NULL,
  groupid numeric NOT NULL,
  CONSTRAINT membergroupid PRIMARY KEY (memberid, groupid),
  CONSTRAINT groupaccount_memberjunction_fk FOREIGN KEY (groupid)
      REFERENCES groupaccount (groupid) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION,
  CONSTRAINT groupmembers_memberjunction_fk FOREIGN KEY (memberid)
      REFERENCES groupmembers (memberid) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION
)
CREATE TABLE groupmembers
(
  memberid integer NOT NULL,
  musername character varying,
  accesslevel boolean DEFAULT true,
  CONSTRAINT memberid PRIMARY KEY (memberid)
)

GroupMembersDAOImpl 删除方法：

@Override
public boolean removeGroupMember(int memberId) {
    session = sessionFactory.getCurrentSession();
    GroupMembers groupMembers = (GroupMembers) session.get(GroupMembers.class, memberId);
    if(!(groupMembers == null)){
        groupMembers.getGroupAccounts().clear();
        session.flush();
        session.delete(groupMembers);
        return true;
    } else {
        return false;
    }
}

GroupAccount 实体：

@Entity
@Table(name="groupaccount")
public class GroupAccount {
 @Id
    @Column(name="groupid")
    @GeneratedValue(strategy = GenerationType.SEQUENCE,generator = "groupaccount_seq_gen")
    @SequenceGenerator(name = "groupaccount_seq_gen",sequenceName = "groupaccount_seq")
    private Long groupId;
 @ManyToMany(cascade = CascadeType.ALL)
    @JoinTable(name = "memberjunction", joinColumns = {@JoinColumn(name = "groupid")},
                inverseJoinColumns = {@JoinColumn(name = "memberid")})
    private Set<GroupMembers> groupMembersSet = new HashSet<>();

    public void setGroupMembersSet(Set<GroupMembers> groupMembersSet){
        this.groupMembersSet = groupMembersSet;
    }

    public Set<GroupMembers> getGroupMembersSet(){
        return this.groupMembersSet;
    }
}

GroupMembers 实体：

@Entity
@Table(name="groupmembers")
public class GroupMembers {

 @Id
    @Column(name="memberid")
    @GeneratedValue(strategy = GenerationType.SEQUENCE,generator = "groupmembers_seq_gen")
    @SequenceGenerator(name = "groupmembers_seq_gen",sequenceName = "groupmembers_seq")
    private int memberid;
 @ManyToMany(mappedBy = "groupMembersSet")
    private Set<GroupAccount> groupAccounts = new HashSet<>();

    public void setGroupAccounts(Set<GroupAccount> groupAccounts){
        this.groupAccounts = groupAccounts;
    }

    public Set<GroupAccount> getGroupAccounts(){
        return this.groupAccounts;
    }
}

最新错误日志：

org.postgresql.util.PSQLException: ERROR: update or delete on table "groupmembers" violates foreign key constraint "groupmembers_memberjunction_fk" on table "memberjunction"
  Detail: Key (memberid)=(2501) is still referenced from table "memberjunction".
    org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2198)
    org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1927)

上面的方法不起作用，正如它所说，有对memberJunction的引用。我如何也可以查询 memberJunction 并删除与 memberId 对应的条目。你能帮忙的话，我会很高兴。非常感谢.. :-)

Answer 1

您需要执行以下操作：

for(GroupAccount groupAccount : groupMembers.getGroupAccounts()) {
    groupAccount.getGroupMembersSet().remove(this);
}
groupMembers.getGroupAccounts().clear();
session.flush();
session.delete(groupMembers);

因为您有一个@ManyToMany 关联，所以关联表在运行时不可用。为此，您可以：

1. 在删除 GroupMembers 之前，从 GroupMembers 中清除 GroupAccounts。确保您刷新，以便确保删除操作doesn't prevent 对可删除实体状态进行更新。

2. 您可以将@ManyToMany 替换为@OneToMany 关联到MemberJunction 实体（其PK/@Id 是从GroupMembers 和GroupAccount 构建的）。这样，您可以将删除从GroupMembers 级联到MemberJunction mappedBy Set，因为它会删除关联表行，而不是实际的GroupAccount 实体。