如何使用双向多对多映射引发以下错误无法添加或更新子行：外键约束失败

Question

我正在尝试通过使用Spring和Hibernate的下表来实现双向多对多映射（question，tags，question_tag）

TABLE用户：

 CREATE TABLE `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `username` varchar(255) DEFAULT NOT NULL,
  `password` varchar(255) DEFAULT NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8;

TABLE问题：

    CREATE TABLE `question` (
  `qid` int(11) NOT NULL AUTO_INCREMENT,
  `questiontitle` varchar(255) DEFAULT NULL,
  `questionbody` varchar(3000) DEFAULT NULL,
  `uid` int(11) NOT NULL,
  `votes` int(11) NOT NULL,
  `created_on` DATETIME NOT NULL DEFAULT NOW(), -- or CURRENT_TIMESTAMP
  PRIMARY KEY (`qid`),
  CONSTRAINT `fk_user_id` FOREIGN KEY (`uid`) REFERENCES `user` (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8;

TABLE标签：

CREATE TABLE `tags` (
  `tagid` int(11) NOT NULL AUTO_INCREMENT,
  `tag` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`tagid`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8;

表格问题标签：

CREATE TABLE `question_tag` (   
    `qid` int(11) NOT NULL,
    `tagid` int(11) NOT NULL,
     PRIMARY KEY (`qid`,`tagid`),
     CONSTRAINT `fk_qt_qid` FOREIGN KEY (`qid`) REFERENCES `question` (`qid`), 
     CONSTRAINT `fk_tags_tagid` FOREIGN KEY (`tagid`) REFERENCES `tags` (`tagid`) 
     ) ENGINE=InnoDB  DEFAULT CHARSET=utf8;

使用的模型是

Question.Java

@Entity
@Table(name="question")
@NamedQuery(name="Question.findAll", query="SELECT q FROM Question q")
public class Question implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int qid;

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name="created_on")
    private Date createdOn;

    private String questionbody;

    private String questiontitle;

    private int uid;

    private int votes;

    //bi-directional many-to-one association to Answer
    @OneToMany(mappedBy="question")
    private Set<Answer> answers = new HashSet<>();

    //bi-directional many-to-one association to QuestionComment
    @OneToMany(mappedBy="question")
    private Set<QuestionComment> questionComments = new HashSet<>();

    //bi-directional many-to-many association to Tag
    @ManyToMany(targetEntity = Tag.class,cascade = { CascadeType.ALL })
    @JoinTable(name = "question_tag",
            joinColumns = { @JoinColumn(name = "qid") },
            inverseJoinColumns = { @JoinColumn(name = "tagid") })
    private Set<Tag> tags;

    //bi-directional many-to-one association to UserQuestionUpvote
    @OneToMany(mappedBy="question")
    private Set<UserQuestionUpvote> userQuestionUpvotes = new HashSet<>();

    public Question() {
    }
}

Tag.Java

@Entity
@Table(name="tags")
@NamedQuery(name="Tag.findAll", query="SELECT t FROM Tag t")
public class Tag implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    private int tagid;

    private String tag;

    //bi-directional many-to-many association to Question
    @ManyToMany(mappedBy = "tags")
    private Set<Question> questions ;

    public Tag() {
    }
}

有一个questionServiceImpl类，它调用questionRepository.save方法。

@Service
public class QuestionServiceImpl implements QuestionService {

    @Autowired
    private QuestionRepository questionRepository;

    @Override
    public void save(Question question) {
        // TODO Auto-generated method stub
        questionRepository.save(question);

    }

}

现在在控制器中，当对QuestionServiceImpl.save的调用被插入时，对表的问题进行了插入，对标签和question_tag进行了调用，但引发了无法添加或更新子行错误

Hibernate: insert into question (created_on, questionbody, questiontitle, uid, votes) values (?, ?, ?, ?, ?)
Hibernate: insert into tags (tag, tagid) values (?, ?)
Hibernate: insert into question_tag (qid, tagid) values (?, ?)
17:43:36.692 [http-bio-9191-exec-5] ERROR o.h.e.j.s.SqlExceptionHelper#129 Cannot add or update a child row: a foreign key constraint fails (`Forum`.`question_tag`, CONSTRAINT `fk_tags_tagid` FOREIGN KEY (`tagid`) REFERENCES `tags` (`tagid`))
Jul 10, 2018 5:43:36 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [dispatcher] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]; constraint [null]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement] with root cause
com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Cannot add or update a child row: a foreign key constraint fails (`Forum`.`question_tag`, CONSTRAINT `fk_tags_tagid` FOREIGN KEY (`tagid`) REFERENCES `tags` (`tagid`))
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:400)
    at com.mysql.jdbc.Util.getInstance(Util.java:383)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:973)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3847)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3783)
    at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2447)
    at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2594)
    at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2545)
    at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1901)
    at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2113)
    at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2049)
    at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2034)