假設，一個農場產出多種植物，具體的某一植物產于某一農場。
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 農場
13 * @作者 fancy
14 * @郵箱 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 * @郵箱 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 測試 :
1 @Test
2 public void createTable(){
3
4 new SchemaExport(new AnnotationConfiguration().configure()).create(true, true);
5 }
執行上面的單元測試，數據庫中生成的表結構圖 :
從上面的圖可以看出，多出了一張中間表，這張中間表是冗余的，它沒有必要存在，它之所以存在，這是因為 hibernate 在默認的情況下，
會將 OneToMany 當成是 ManyToMany 的一種特殊情況，故而生成了一張中間表。想要去掉這張中間表只需要加上 @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 }
再次執行單元測試 ( 注意要手動刪除之前生成的表，因為有中間表的存在，中間表參考了與它關聯的主表，這種情況下 hibernate 無法幫我們刪除表 )
級聯保存 :
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 }
28 }
posted on 2012-10-21 08:04
fancydeepin 閱讀(3971)
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