如何连接 JPA 实体类？ (one-to-many/many-to-one)

Question

如果有这两个实体：

@Entity
@Table(name = "CUSTOMER")
public class Customer {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Column(name = "FIRST_NAME") //Attributes in the entitiy
    private String firstName;

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

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

和

@Entity
@Table(name = "ADDRESS")
public class Address {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Column(name = "STREET_NAME") //Attributes in the entitiy
    private String streetName;

    @Column(name = "HOUSE_NUMBER")
    private int houseNumber;

我现在如何连接这两个实体并定义 Customer 有很多 Addresses？

Answer 1

你应该有一个 xml 文件来定义这个实体指的是哪个 table，然后你可以在那里添加它们的字段和关系。

示例：

<entity class="class" access="FIELD">
<table name="class_table" />

 <attributes>
    <id name="classId">
        <column name="class_id" />
    </id>

    <basic name="className">
        <column name="case_name" />
    </basic>

<!-- Foreign key many-to-one -->
    <many-to-one name="classType" fetch="LAZY">
        <join-column name="class_type_id" referenced-column-name="class_type_id" />
    </many-to-one>

<!-- Foreign key one-to-many -->
    <one-to-many name="class_list"  mapped-by="class" fetch="LAZY" />
 </attributes>
 </entity>

Answer 2

您可以在 Internet 上轻松找到大量此类示例。我强烈建议看看 ObjectDB website.

对于这个特定的场景，您可以如下定义它们：

@Entity
@Table(name = "CUSTOMER")
public class Customer {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Column(name = "FIRST_NAME") //Attributes in the entity
    private String firstName;

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

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

    @OneToMany(mappedBy = "customer")
    private List<Address> addresses;
}

并在地址 class 中引用您的客户：

@Entity
@Table(name = "ADDRESS")
public class Address {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Column(name = "STREET_NAME") //Attributes in the entitiy
    private String streetName;

    @Column(name = "HOUSE_NUMBER")
    private int houseNumber;

    @ManyToOne(fetch = FetchType.LAZY)
    private Customer customer;
}

编辑： 根据您对多对多关系示例的请求，您可以通过以下方式为您的客户和地址实体实现它：

首先，您应该有一个具有两个外键的中间 table，一个指向客户 table，另一个指向地址。假设你将这个中间体命名为 table customer_address，那么它的数据将是这样的：

select * from customer_address;

customer_id | address_id
------------------------
    12      |     15
    12      |     14
    12      |     13
    2       |     15
    2       |     13
    19      |     11

现在您应该更新注释并告诉 JPA 使用此 table 来推断实体之间的多对多关系。

客户class：

@Entity
@Table(name = "CUSTOMER")
public class Customer {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Column(name = "FIRST_NAME") //Attributes in the entity
    private String firstName;

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

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

    @ManyToMany(mappedBy = "customer")
    @JoinTable(name = "customer_address", 
        joinColumns = @JoinColumn(name = "customer_id", referencedColumnName = "id"),
        inverseJoinColumns = @JoinColumn(name = "address_id", referencedColumnName = "id"))
    private List<Address> addresses;
}

@Entity
@Table(name = "ADDRESS")
public class Address {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Column(name = "STREET_NAME") //Attributes in the entitiy
    private String streetName;

    @Column(name = "HOUSE_NUMBER")
    private int houseNumber;

    @ManyToMany(mappedBy = "customer")
    @JoinTable(name = "customer_address", 
        joinColumns = @JoinColumn(name = "address_id", referencedColumnName = "id"),
        inverseJoinColumns = @JoinColumn(name = "customer_id", referencedColumnName = "id"))
    private List<Customer> customer;
}

在此代码中，注释用于向 JPA 提供以下信息：

@ManyToMany：表示关系的类型
@JoinTable：用于指定持有实际关系数据的table，具有以下属性：
- joinColumns：定义中间 table 中的列，该列引用包含 @ManyToMany 注释的实体 class。在 Customer class 中，它定义了作为 customer table 的外键的列，在 Address class 中，它定义了作为外键的列是 address table.
- inverseJoinColumns：与joinColumn属性正好相反。定义中间 table 中的列，该列引用 @ManyToMany 注释另一侧的实体 class。在 Customer class 中，它定义了作为 address table 的外键的列，在 Address class 中，它定义了作为外键的列是 customer table.

我知道我的描述不尽如人意readable/understandable，但你知道@JoinTable注解的描述并不容易。阅读示例代码本身并匹配 column/property 名称以找出每个名称的含义将是一种更简单的方法。您也可以在 ObjectWeb 网站上查看（也包括示例）。

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如何连接 JPA 实体类？ (one-to-many/many-to-one)