Hibernate 多對一與一對多關聯映射實戰：從 XML 到 CRUD 完整流程

前言

關聯映射是 Hibernate 核心能力。本文通過 多對一（多員工共用一地址）與 一對多（一員工多證書）兩大經典場景，完整演示實體設計、XML 映射、表結構、CRUD 測試代碼。

場景一：多對一 —— 多員工共用地址

1. 實體類

public class Employee {
    private Integer id;
    private String firstName, lastName;
    private Integer salary;
    private Address address;  // 多對一關聯
    // Getter/Setter
}

public class Address {
    private Integer id;
    private String street, city, state, zipcode;
    // Getter/Setter
}

2. 表結構

CREATE TABLE EMPLOYEE (
    id INT PRIMARY KEY AUTO_INCREMENT,
    first_name VARCHAR(20), last_name VARCHAR(20), salary INT,
    address_id INT NOT NULL,
    FOREIGN KEY (address_id) REFERENCES ADDRESS(id)
);

CREATE TABLE ADDRESS (
    id INT PRIMARY KEY AUTO_INCREMENT,
    street_name VARCHAR(40), city_name VARCHAR(40),
    state_name VARCHAR(40), zipcode VARCHAR(10)
);

3. 映射文件

<class name="Employee" table="EMPLOYEE">
    <id name="id" column="id"><generator class="native"/></id>
    <property name="firstName" column="first_name"/>
    <property name="lastName" column="last_name"/>
    <property name="salary" column="salary"/>
    <many-to-one name="address" column="address_id" class="Address" not-null="true"/>
</class>

<class name="Address" table="ADDRESS">
    <id name="id" column="id"><generator class="native"/></id>
    <property name="street" column="street_name"/>
    <property name="city" column="city_name"/>
    <property name="state" column="state_name"/>
    <property name="zipcode" column="zipcode"/>
</class>

關鍵：<many-to-one> 定義關聯端，column 指定外鍵列，class 指定關聯實體，not-null="true" 保證外鍵非空。

4. CRUD 測試

// 保存地址（先保存被引用端）
Address addr = new Address("Kondapur", "Hyderabad", "AP", "532");
session.save(addr);

// 保存員工（關聯地址）
Employee e1 = new Employee("Manoj", "Kumar", 4000, addr);
Employee e2 = new Employee("Dilip", "Kumar", 3000, addr);
session.save(e1); session.save(e2);

// 查詢驗證
List<Employee> list = session.createQuery("from Employee").list();
for (Employee e : list) {
    System.out.println(e.getFirstName() + " " + e.getAddress().getCity());
}
// 輸出：Manoj Hyderabad / Dilip Hyderabad

場景二：一對多 —— 員工擁有多證書

1. 實體類

public class Employee {
    private Integer id;
    private String firstName, lastName;
    private Integer salary;
    private Set<Certificate> certificates = new HashSet<>(); // 一對多
    // Getter/Setter
}

public class Certificate {
    private Integer id;
    private String name;
    // Getter/Setter
}

2. 表結構（同場景一 EMPLOYEE 表，新增 CERTIFICATE 表）

CREATE TABLE CERTIFICATE (
    id INT PRIMARY KEY AUTO_INCREMENT,
    certificate_name VARCHAR(30),
    employee_id INT,
    FOREIGN KEY (employee_id) REFERENCES EMPLOYEE(id)
);

3. 映射文件

<class name="Employee" table="EMPLOYEE">
    <id name="id" column="id"><generator class="native"/></id>
    <set name="certificates" cascade="all">
        <key column="employee_id"/>
        <one-to-many class="Certificate"/>
    </set>
    <property name="firstName" column="first_name"/>
    <property name="lastName" column="last_name"/>
    <property name="salary" column="salary"/>
</class>

<class name="Certificate" table="CERTIFICATE">
    <id name="id" column="id"><generator class="native"/></id>
    <property name="name" column="certificate_name"/>
</class>

關鍵：<set> 保證唯一性，<key> 定義外鍵，<one-to-many> 聲明集合元素類型，cascade="all" 級聯保存/刪除。

4. CRUD 測試

// 保存員工及證書
Employee emp = new Employee("Manoj", "Kumar", 4000);
Set<Certificate> certs = new HashSet<>();
certs.add(new Certificate("MCA"));
certs.add(new Certificate("MBA"));
certs.add(new Certificate("PMP"));
emp.setCertificates(certs);
session.save(emp);  // 級聯保存證書

// 查詢並遍歷
List<Employee> list = session.createQuery("from Employee").list();
for (Employee e : list) {
    for (Certificate c : e.getCertificates()) {
        System.out.println(c.getName());
    }
}
// 輸出：MBA, PMP, MCA（Set 無序）

核心對比總結

最佳實踐建議

1. 雙向關聯：一對多建議在 Certificate 加 @ManyToOne / <many-to-one> 反向引用，便於從多方導航。
2. cascade 謹慎選擇：all 含刪除，刪除員工會連帶刪除證書；若證書獨立生命週期用 save-update。
3. 懶加載：集合預設懶加載，遍歷時需在 Session 開啟狀態，或用 fetch="join" / HQL join fetch。
4. 現代化：新項目優先用 JPA 註解（@OneToMany/@ManyToOne），XML 僅用於歷史遷移。