一個實體簡單的說就是在資料庫中擁有一個表格，並擁有自已的資料庫識別（Database identity）。
一個簡單的實體與實體間之關係為多對一的關係，例如在學校宿舍中，使用者與房間的關係就是多對一的關係，多個使用者可以居住於一個房間。
如上圖所示的，可以藉由room_id讓使用者與房間產生關聯，您可以如下建立user與room表格：
create table room (
id bigint not null auto_increment,
address varchar(255),
primary key (id)
)
create table user (
id bigint not null auto_increment,
name varchar(255),
room_id bigint,
primary key (id)
)
id bigint not null auto_increment,
address varchar(255),
primary key (id)
)
create table user (
id bigint not null auto_increment,
name varchar(255),
room_id bigint,
primary key (id)
)
用程式來表示的話，首先看看User類別：
- User.java
package onlyfun.caterpillar;
public class User {
private Long id;
private String name;
private Room room;
public User() {}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Room getRoom() {
return room;
}
public void setRoom(Room room) {
this.room = room;
}
}User類別中有一room屬性，將參考至Room實例，多個User實例可共同參考一個Room實例，Room類別設計如下：
- Room.java
package onlyfun.caterpillar;
public class Room {
private Long id;
private String address;
public Room() {}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
}在映射文件方面，先來看看Room.hbm.xml：
- Room.hbm.xml
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping
PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="onlyfun.caterpillar.Room" table="room">
<id name="id" column="id">
<generator class="native"/>
</id>
<property name="address" column="address" />
</class>
</hibernate-mapping>沒什麼，很簡單的一個映射文件，而在User.hbm.xml中，使用<many-to-one>標籤來映射多對一關係：
- User.hbm.xml
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping
PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="onlyfun.caterpillar.User" table="user">
<id name="id" column="id">
<generator class="native"/>
</id>
<property name="name" column="name"/>
<many-to-one name="room"
column="room_id"
class="onlyfun.caterpillar.Room"
cascade="all"
outer-join="true"/>
</class>
</hibernate-mapping>
在<many-to-one>的設定中，cascade表示主控方（User）進行save-pdate、delete等 相關操作時，被控 方（Room）是否也一併進行相關操作，簡單的說，也就是您儲存或更新User實例時，被參考到的Room實例是否一併對資料庫發生儲存或操作，設定為 all，表示主控方任何操作，被控方也進行對應操作。
一個儲存的例子如下：
Room
room1 = new Room();
room1.setAddress("NTU-M8-419");
Room room2 = new Room();
room2.setAddress("NTU-G3-302");
User user1 = new User();
user1.setName("bush");
user1.setRoom(room1);
User user2 = new User();
user2.setName("caterpillar");
user2.setRoom(room1);
User user3 = new User();
user3.setName("momor");
user3.setRoom(room2);
Session session = HibernateUtil.getSessionFactory().openSession();
Transaction tx = session.beginTransaction();
session.save(user1); // 主控方操作，被控方也會對應操作
session.save(user2);
session.save(user3);
tx.commit();
session.close();
room1.setAddress("NTU-M8-419");
Room room2 = new Room();
room2.setAddress("NTU-G3-302");
User user1 = new User();
user1.setName("bush");
user1.setRoom(room1);
User user2 = new User();
user2.setName("caterpillar");
user2.setRoom(room1);
User user3 = new User();
user3.setName("momor");
user3.setRoom(room2);
Session session = HibernateUtil.getSessionFactory().openSession();
Transaction tx = session.beginTransaction();
session.save(user1); // 主控方操作，被控方也會對應操作
session.save(user2);
session.save(user3);
tx.commit();
session.close();
關於cascade，可以進一步參考 cascade 的意義 。
資料庫中將儲存以下的內容：
mysql> select *
from user;
+----+-------------+-----------+
| id | name | room_id |
+----+-------------+-----------+
| 1 | bush | 1 |
| 2 | caterpillar | 1 |
| 3 | momor | 2 |
+----+-------------+-----------+
3 rows in set (0.00 sec)
mysql> select * from room;
+----+-------------------+
| id | address |
+----+-------------------+
| 1 | NTU-M8-419 |
| 2 | NTU-G3-302 |
+----+-------------------+
2 rows in set (0.00 sec)
+----+-------------+-----------+
| id | name | room_id |
+----+-------------+-----------+
| 1 | bush | 1 |
| 2 | caterpillar | 1 |
| 3 | momor | 2 |
+----+-------------+-----------+
3 rows in set (0.00 sec)
mysql> select * from room;
+----+-------------------+
| id | address |
+----+-------------------+
| 1 | NTU-M8-419 |
| 2 | NTU-G3-302 |
+----+-------------------+
2 rows in set (0.00 sec)
在查詢時的例子如下：
Session session =
HibernateUtil.getSessionFactory().openSession();
User user = (User) session.load(User.class, new Long(1));
System.out.println(user.getName());
System.out.println(user.getRoom().getAddress());
session.close();
User user = (User) session.load(User.class, new Long(1));
System.out.println(user.getName());
System.out.println(user.getRoom().getAddress());
session.close();
在設定outer-join為true的情況下，Hibernate將使用以下的SQL一次查詢所有的資料：
Hibernate:
select
user0_.id as id0_1_,
user0_.name as name0_1_,
user0_.room_id as room3_0_1_,
room1_.id as id1_0_,
room1_.address as address1_0_
from
user user0_
left outer join
room room1_
on user0_.room_id=room1_.id
where
user0_.id=?
select
user0_.id as id0_1_,
user0_.name as name0_1_,
user0_.room_id as room3_0_1_,
room1_.id as id1_0_,
room1_.address as address1_0_
from
user user0_
left outer join
room room1_
on user0_.room_id=room1_.id
where
user0_.id=?
在不設定outer-join為true的情況下，Hibernate則使用以下的SQL分別查詢user與room表格：
Hibernate:
select
user0_.id as id0_0_,
user0_.name as name0_0_,
user0_.room_id as room3_0_0_
from
user user0_
where
user0_.id=?
Hibernate:
select
room0_.id as id1_0_,
room0_.address as address1_0_
from
room room0_
where
room0_.id=?
select
user0_.id as id0_0_,
user0_.name as name0_0_,
user0_.room_id as room3_0_0_
from
user user0_
where
user0_.id=?
Hibernate:
select
room0_.id as id1_0_,
room0_.address as address1_0_
from
room room0_
where
room0_.id=?