java - 在 hibernate 中仅将属性设置为外键 int 就足够了吗？

Question

我想在hibernate java类中的Mysql表之间建立外键一对一关系。

数据库:

表1:

Id   int         PK
name varchar(10) NN

表2:

Id      int         PK
name    varchar(10) NN
tabe1Id int         FK

代码:

JavaClass1

@Entity
@Table(name="table1")
public class table1{

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="Id")
    private int Id;

    @Column(name="name")
    private String name;
}

JavaClass2

@Entity
@Table(name="table2")
public class table2{

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="Id")
    private int Id;

    @Column(name="name")
    private String name;

    @Id
    @Column(name="table1Id")
    private int JavaClass1Id;
}

这样写就够了吗？将 JavaClass1Id 作为整数存储在 JavaClass2 中，而不在 hibernate 中与另一个类建立关系？

Answer 1

正如人们已经指出的那样，您正在寻找 @OneToOne 和 JPA 指南。这是对此的基本方法:

类表1:

@Entity
@Table(name="table1")
public class table1{

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="Id")
    private int Id;

    @Column(name="name")
    private String name;

    // omitting constructors, getters, setters, toString, etc.
}

类表2:

@Entity
@Table(name="table2")
public class table2{

    @Id
    @GeneratedValue(strategy= GenerationType.IDENTITY)
    @Column(name="Id")
    private int Id;

    @Column(name="name")
    private String name;

    @OneToOne // using default values for db schema
    private table1 t1; // notice how you have an actual reference to your object

    // omitting constructors, getters, setters, toString, etc.
}

演示:

public void demo() {
    table1 t1 = new table1("my table1 object");
    t1 = t1Repo.save(t1);

    table2 t2 = new table2("my table2 object", t1);
    t2 = t2Repo.save(t2);

    for (table2 t : t2Repo.findAll()) {
        log.info("retrieved t2 object " + t);
    }
}

打印:

retrieved t2 object table2{Id=1, name='my table2 object', t1=table1{Id=1, name='my table1 object'}}

这是相应的 MySQL 的样子:

MariaDB [so41334487]> show tables;
+----------------------+
| Tables_in_so41334487 |
+----------------------+
| table1               |
| table2               |
+----------------------+
2 rows in set (0.00 sec)

MariaDB [so41334487]> describe table1;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| id    | int(11)      | NO   | PRI | NULL    | auto_increment |
| name  | varchar(255) | YES  |     | NULL    |                |
+-------+--------------+------+-----+---------+----------------+
2 rows in set (0.06 sec)

MariaDB [so41334487]> describe table2;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| id    | int(11)      | NO   | PRI | NULL    | auto_increment |
| name  | varchar(255) | YES  |     | NULL    |                |
| t1_id | int(11)      | YES  | MUL | NULL    |                |
+-------+--------------+------+-----+---------+----------------+
3 rows in set (0.01 sec)

MariaDB [so41334487]> select * from table1;
+----+------------------+
| id | name             |
+----+------------------+
|  1 | my table1 object |
+----+------------------+
1 row in set (0.00 sec)

MariaDB [so41334487]> select * from table2;
+----+------------------+-------+
| id | name             | t1_id |
+----+------------------+-------+
|  1 | my table2 object |     1 |
+----+------------------+-------+
1 row in set (0.00 sec)

这是从 SHOW CREATE TABLE table2 中提取的相应 CREATE 语句:

CREATE TABLE `table2` (
    `id` int(11) NOT NULL AUTO_INCREMENT,
    `name` varchar(255) DEFAULT NULL,
    `t1_id` int(11) DEFAULT NULL,
    PRIMARY KEY (`id`),
    KEY `FK30i46miycdu2aqony71wj8kva` (`t1_id`),
    CONSTRAINT `FK30i46miycdu2aqony71wj8kva` FOREIGN KEY (`t1_id`) REFERENCES `table1` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=latin1 

那里有很多高质量的内容，包括关于 @OneToOne 的书籍和视频讲座以及相应的陷阱，但是如果您正在寻找快速提示，您可以开始 in the WikiBook on JavaPersistence 。另请记住，如果您在(一系列)对象之间创建循环引用，则链接 FetchType.EAGER 可能会导致您将整个数据库加载到内存中。