hibernate annotation 之 一对多单向外键关联
假设，一个农场产出多种植物，具体的某一植物产于某一农场。
1 package net.yeah.fancydeepin.po;
2
3 import java.io.Serializable;
4 import java.util.Set;
5 import javax.persistence.Column;
6 import javax.persistence.Entity;
7 import javax.persistence.GeneratedValue;
8 import javax.persistence.Id;
9 import javax.persistence.OneToMany;
10 /**
11 * -----------------------------------------
12 * @描述 农场
13 * @作者 fancy
14 * @邮箱 fancydeepin@yeah.net
15 * @日期 2012-10-21 <p>
16 * -----------------------------------------
17 */
18 @Entity
19 public class Farm implements Serializable{
20
21 private static final long serialVersionUID = 1L;
22
23 private Integer id;
24 private String name;
25 private Set<Plant> plantSet;
26
27 @Id
28 @GeneratedValue
29 public Integer getId() {
30 return id;
31 }
32 @Column(length = 18, nullable = false)
33 public String getName() {
34 return name;
35 }
36 @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
37 public Set<Plant> getPlantSet() {
38 return plantSet;
39 }
40 public void setId(Integer id) {
41 this.id = id;
42 }
43 public void setName(String name) {
44 this.name = name;
45 }
46 public void setPlantSet(Set<Plant> plantSet) {
47 this.plantSet = plantSet;
48 }
49 }
1 package net.yeah.fancydeepin.po;
2
3 import java.io.Serializable;
4 import javax.persistence.Column;
5 import javax.persistence.Entity;
6 import javax.persistence.GeneratedValue;
7 import javax.persistence.Id;
8 /**
9 * -----------------------------------------
10 * @描述 植物
11 * @作者 fancy
12 * @邮箱 fancydeepin@yeah.net
13 * @日期 2012-10-21 <p>
14 * -----------------------------------------
15 */
16 @Entity
17 public class Plant implements Serializable{
18
19 private static final long serialVersionUID = 1L;
20
21 private Integer id;
22 private String name;
23
24 @Id
25 @GeneratedValue
26 public Integer getId() {
27 return id;
28 }
29 @Column(length = 18, nullable = false)
30 public String getName() {
31 return name;
32 }
33 public void setId(Integer id) {
34 this.id = id;
35 }
36 public void setName(String name) {
37 this.name = name;
38 }
39
40 }
Junit 测试 :
1 @Test
2 public void createTable(){
3
4 new SchemaExport(new AnnotationConfiguration().configure()).create(true, true);
5 }
执行上面的单元测试，数据库中生成的表结构图 :
从上面的图可以看出，多出了一张中间表，这张中间表是冗余的，它没有必要存在，它之所以存在，这是因为 hibernate 在默认的情况下，
会将 OneToMany 当成是 ManyToMany 的一种特殊情况，故而生成了一张中间表。想要去掉这张中间表只需要加上 @JoinColumn 注解即可 :
1 @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
2 @JoinColumn(name = "farm_id")
3 public Set<Plant> getPlantSet() {
4 return plantSet;
5 }
再次执行单元测试 ( 注意要手动删除之前生成的表，因为有中间表的存在，中间表参考了与它关联的主表，这种情况下 hibernate 无法帮我们删除表 )
级联保存 :
1 private static Session session;
2
3 @BeforeClass
4 public static void beforeClass(){
5
6 session = new AnnotationConfiguration().configure().buildSessionFactory().getCurrentSession();
7 }
8
9 @Test
10 public void insert(){
11 try {
12 Plant tomato = new Plant();
13 tomato.setName("番茄");
14 Plant cabbage = new Plant();
15 cabbage.setName("卷心菜");
16 Set<Plant> plantSet = new HashSet<Plant>();
17 plantSet.add(tomato);
18 plantSet.add(cabbage);
19 Farm farm = new Farm();
20 farm.setName("fancy-farm");
21 farm.setPlantSet(plantSet);
22 session.beginTransaction();
23 session.persist(farm);
24 session.getTransaction().commit();
25 } catch (Exception e) {
26 e.printStackTrace();
27 }
