jpa - @OneToOne 单向和双向

Question

我有两个例子，第一个是@OneToOne单向映射，第二个是双向映射。在单向映射中，拥有方表必须包含引用对方表id的连接列；那么在双向中，它们都必须包含彼此的外键列。但是使用自动生成策略生成数据库模式后，两个示例对数据库模式的效果相同。单向映射是正常的，但双向示例只包含一个外键列，但必须涉及彼此的外键!

单向映射

@Entity
public class Customer43 {

@Id
@GeneratedValue
private Long id;
private String firstName;
private String lastName;
private String email;
private String phoneNumber;
@OneToOne
@JoinColumn(name = "address_fk")
private Address43 address;

// Getters, Setters and Constructors.
}

@Entity
public class Address43 {

@Id
@GeneratedValue
private Long id;
private String street1;
private String street2;
private String city;
private String state;
private String zipcode;
private String country;

// Getters, Setters and Constructors.
}

双向映射

@Entity
public class Customer44 {

@Id
@GeneratedValue
private Long id;
private String firstName;
private String lastName;
private String email;
private String phoneNumber;
@OneToOne
@JoinColumn(name = "address_fk")
private Address43 address;

// Getters, Setters and Constructors.
}

@Entity
public class Address44 {

@Id
@GeneratedValue
private Long id;
private String street1;
private String street2;
private String city;
private String state;
private String zipcode;
private String country;
@OneToOne(mappedBy = "address")
private Customer44 customer;

// Getters, Setters and Constructors.
}

为什么数据库模式输出相同以及为什么双向映射表现得像单向？

Answer 1

因为你不明白双向 OneToOne 是如何映射的。您不需要两个外键。单个表足以在两个表之间执行双向联接:

select a.* from address a inner join customer c on c.addressId = a.id;
select c.* from customer c inner join address a on c.addressId = a.id;

关联是单向或双向的事实不会改变表的链接方式。它只是更改映射，并允许在两个方向上导航关联。

在双向关联中，您总是有一个拥有方(它告诉如何映射关联，使用哪个连接列)和一个反面，您只需说:我是关联的另一方，即目标实体中此字段(mappedBy 属性值中的字段)映射。