hibernate 多對一關係映射

實體是Employee和Department，它們之間是多對一的關係。

Department類：

public class Department {
    private int id;
    private String name;

    public Department() {
    }
    public Department(String name) {
        this.name = name;
    }
    // getters and setters are omitted
}

Employee類：

public class Employee {
    private int id;
    private String name;
    private Department department;

    public Employee() {
    }
    public Employee(String name) {
        this.name = name;
    }
    // getters and setters are omitted

Department.hbm.xml:

<hibernate-mapping
    package="com.john.myhibernate.domain">

    <class name="Department">
        <id name="id">
            <generator class="native"/>
        </id>
        <property name="name" length="20" not-null="true"/>
    </class>
</hibernate-mapping>

Employee.hbm.xml:

<hibernate-mapping package="com.john.myhibernate.domain">

<class name="Employee">
    <id name="id">
        <generator class="native"/>
    </id>
    <property name="name" length="20" not-null="true"/>
    <many-to-one name="department" column="department_id" class="Department" fetch="select"/>
</class>
</hibernate-mapping>

many-to-one沒有inverse屬性，因爲關係的維護是many的一方，不可能放棄對關係的維護。
many-to-one的lazy屬性有三個取值：false, proxy, no-proxy。

1. 測試cascade屬性：

public void testSaveCascade() {
    Session s = null;
    Transaction tx = null;

    Department depart = new Department();
    depart.setName("FCI");

    Employee em1 = new Employee("John");
    em1.setDepartment(depart);

    Employee em2 = new Employee("Lucy");
    em2.setDepartment(depart);

    try {
        s = HibernateUtil.getSession();
        tx = s.beginTransaction();
        s.save(em1);
        s.save(em2);
        tx.commit();
    } catch (HibernateException e) {
        tx.rollback();
        e.printStackTrace();
    } finally {
        if (s != null)
            s.close();
    }
}

結果是報org.hibernate.TransientObjectException異常，因爲沒有保存Department實例。

可以加cascade屬性，解決問題：

<many-to-one name="department" column="department_id" class="Department" fetch="select" cascade="save-update"/>

2. 測試fetch

Session s = null;

s = HibernateUtil.getSession();
Employee em = (Employee) s.get(Employee.class, 2);
System.out.println(em.getName());
System.out.println(em.getDepartment());

查詢語句如下：
Hibernate: select employee0_.id as id1_0_, employee0_.name as name1_0_, employee0_.department as department1_0_, employee0_.skill as skill1_0_, employee0_.sell as sell1_0_, employee0_.type as type1_0_ from Employee employee0_ where employee0_.id=?

Hibernate: select department0_.id as id0_0_, department0_.name as name0_0_ from Department department0_ where department0_.id=?
因爲fetch設置爲select，所以對每個實體，都分別用一個SELECT語句

如果把fetch設置爲join，也就是連表查詢，只使用一個SELECT語句。如下：
Hibernate: select employee0_.id as id1_1_, employee0_.name as name1_1_, employee0_.department as department1_1_, employee0_.skill as skill1_1_, employee0_.sell as sell1_1_, employee0_.type as type1_1_, department1_.id as id0_0_, department1_.name as name0_0_ from Employee employee0_ left outer join Department department1_ on employee0_.department=department1_.id where employee0_.id=?

3. 測試lazy
當fetch爲select時，設置lazy爲proxy或者no-proxy。

<many-to-one name="department" column="department_id" class="Department" fetch="select" cascade="save-update" lazy="no-proxy"/>

Session s = null;

s = HibernateUtil.getSession();
Employee em = (Employee) s.get(Employee.class, 2);
s.close();
System.out.println(em.getName());
System.out.println(em.getDepartment());

結果是報org.hibernate.LazyInitializationException異常。
因爲fetch爲select，而且lazy爲proxy或者no-proxy，所以開始僅僅查詢Employee，當需要用SELECT語句查詢Department時，Session已經關閉。

解決辦法：
1. 設置lazy爲false，hibernate會第一時間把Employee和Department查詢出來。
   如果fetch爲select，使用兩個SELECT查詢語句。
   如果fetch爲join，使用一個SELECT連表查詢語句。
2. 設置fetch爲join，這時不管lazy的取值，hibernate會進行連表查詢，把兩個實體都查詢出來。