JPA 添加多对多报错 Cannot add or update a child row: a foreign key constraint fails

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报错就是这个

Caused by: java.sql.SQLIntegrityConstraintViolationException: Cannot add or update a child row: a foreign key constraint fails (`cms`.`user_role`, CONSTRAINT `FK859n2jvi8ivhui0rl0esws6o` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`))
	at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:117)
	at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97)
	at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122)
	at com.mysql.cj.jdbc.ClientPreparedStatement.executeInternal(ClientPreparedStatement.java:974)
	at com.mysql.cj.jdbc.ClientPreparedStatement.executeUpdateInternal(ClientPreparedStatement.java:1113)
	at com.mysql.cj.jdbc.ClientPreparedStatement.executeUpdateInternal(ClientPreparedStatement.java:1061)
	at com.mysql.cj.jdbc.ClientPreparedStatement.executeLargeUpdate(ClientPreparedStatement.java:1381)
	at com.mysql.cj.jdbc.ClientPreparedStatement.executeUpdate(ClientPreparedStatement.java:1046)
	at com.zaxxer.hikari.pool.ProxyPreparedStatement.executeUpdate(ProxyPreparedStatement.java:61)
	at com.zaxxer.hikari.pool.HikariProxyPreparedStatement.executeUpdate(HikariProxyPreparedStatement.java)
	at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:175)

大概就是字面意思把。无法升级。因为找不到对应的外键。但是我检查了 数据库。都有对应。后来发现是因为

JPA 实体类 主键上面没有规定 生成策略  

    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    @Column(name = "id")
    public long getId() {
        return id;
    }

 在 变量上面或者get方法上面添加 @GeneratedValue 注解。 之前遇到一次这个错误。当时忘记记录了。导致在这个上面又耽误了一会。特此记录