第九章 关系映射 一对多关联映射

如：department与employee
employee中有一个department_id的外键
Department:
public class Department implements Serializable {
private Integer id;
private String name;
private Set<Employee> employees;

public Set<Employee> getEmployees() {
return employees;
}

public void setEmployees(Set<Employee> employees) {
this.employees = employees;
}

public Department() {
}

public Integer getId() {
return id;
}

public void setId(Integer id) {
this.id = id;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}
}

Employee:

public class Employee implements Serializable {
private Integer id;
private String name;

public Employee() {
}

public Integer getId() {
return id;
}

public void setId(Integer id) {
this.id = id;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}
}

Department.hbm.xml:(与普通的映射文件一样)
<hibernate-mapping>
<class name="cn.framelife.hibernate.entity.Department" table="department"
catalog="hibernate">
<id name="id" type="java.lang.Integer">
<column name="id" />
</id>
<property name="name" type="java.lang.String">
<column name="name" length="45" not-null="true" />
</property>
<set name="employees" inverse="false" cascade="all">
<key column="department_id"></key>
<one-to-many class="cn.framelife.hibernate.entity.Employee"/>
</set>
</class>
</hibernate-mapping>

Employee.hbm.xml:

<hibernate-mapping>
<class name="cn.framelife.hibernate.entity.Employee" table="employee"
catalog="hibernate">
<id name="id" type="java.lang.Integer">
<column name="id" />
<generator class="native"></generator>
</id>
<property name="name" type="java.lang.String">
<column name="name" length="45" not-null="true" />
</property>
</class>
</hibernate-mapping>

增加：
transaction = session.beginTransaction();
Department department = new Department();
department.setName("zzza");
Set<Employee> set = new HashSet<Employee>();
for(int i=0; i < 5; i++){
Employee employee = new Employee();
employee.setName("zz"+i);
set.add(employee);
}
department.setEmployees(set);
session.save(department);
transaction.commit();

查询：

查询department的时候可以得到外键关联的所有employee对象。

注意事项：

a、错误提示:Field'department_id'
doesn't have a default value

   数据表中把"department_id"设成可以为空的，但是Hibernate先执行的是:
因为hibernate执行的顺序是这样的：
Hibernate:insert into hibernate.department (name) values (?)
Hibernate:insert into hibernate.employee (name) values (?)
Hibernate:insert into hibernate.employee (name) values (?)
Hibernate:insert into hibernate.employee (name) values (?)
Hibernate:insert into hibernate.employee (name) values (?)
Hibernate:insert into hibernate.employee (name) values (?)
Hibernate:update hibernate.employee set department_id=? where id=?
Hibernate:update hibernate.employee set department_id=? where id=?
Hibernate:update hibernate.employee set department_id=? where id=?
Hibernate:update hibernate.employee set department_id=? where id=?
Hibernate:update hibernate.employee set department_id=? where id=?
department_id外键是作为一个后来才更新的存在。所有不能设置为非空的。

b、Department.hbm.xml中要设置cascade="all"(级联)，或其它有效值，不然，在保存Department对象时，相关的Employee对象不会被保存。(none,all,save-update,delete..)

c、Department.hbm.xml中set标签的inverse属性不能设置为"true"（放弃维护关联关系），inverse的默认值是"false"，所以不加inverse也可以。看书上说:在一对多的关联关系实现中，最好设置inverse="true"，将有助于性能的改善。所以一开始就用了inverse="true"，User和Card对象都分别正确写入数据库了，但是就是departmentID字段没有被自动写入。

d、多对一与一对多可以一起用，形成双向关系。
      多对一映射的使用：http://blog.csdn.net/p_3er/article/details/9036759

e.one-to-many、many-to-one、many-to-many懒加载分析：

必须同时满足下面的两个条件时才能实现懒散加载：

1).lazy!=false(lazy缺省方式就!=false，lazy是=proxy)

2).fetch=select(fetch缺省方式即为select)