hibernate 自然键 和 复合自然键

必须提醒你：当应用程序继承现有的遗留数据库Schema时，通常应该尽可能地对现有的Schema少做一些改变。你
对Schema所做的每一处改变都可能破坏访问数据库的其他现有应用程序。现有数据的迁移可能很昂贵，这也是你要考虑
的问题。一般来说，构建一个新的应用程序不可能不对现有的数据模型做任何改变--新应用程序通常意味着
额外的业务需求，自然需要数据库Schema的演变。




处理主键：
不幸的是，遗留的数据库很多使用了自然主键、复合键，这样在业务需求改变时，很难重构数据库。


1、映射自然键
如你遇到的User表，可能userName是一个自燃键。那么你就需要告诉hibernate，标示符是在对象保存之前由应用
分配的一个自燃键：

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1. <classname="User"table="USER">
2. <!-- PRIMARY KEY(`USER_NAME`) -->
3. <id name="userName"column="USER_NAME">
4. <generatorclass="assigned"></generator>
5. </id>
6. <property name="password"></property>
7. </class>

JPA实现：

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1. @Entity
2. publicclassUserimplementsSerializable {
3. @Id
4. @Column(name="USER_NAME")
5. privateString userName;
6. privateString password;

2、映射复合自然健
复合主键单独抽取出来一个实体来表示主键会比较优雅
代码：
UserID：要重装hashCode

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1. publicclassUserIdimplementsSerializable {
2. privateString userCode;
3. privateString userName;
5. publicUserId() {
6. super();
7. }
9. publicUserId(String userCode, String userName) {
10. super();
11. this.userCode = userCode;
12. this.userName = userName;
13. }
15. publicString getUserCode() {
16. returnuserCode;
17. }
19. publicvoidsetUserCode(String userCode) {
20. this.userCode = userCode;
21. }
23. publicString getUserName() {
24. returnuserName;
25. }
27. publicvoidsetUserName(String userName) {
28. this.userName = userName;
29. }
31. @Override
32. publicinthashCode() {
33. finalintprime =31;
34. intresult =1;
35. result = prime * result
36. + ((userCode ==null) ?0: userCode.hashCode());
37. result = prime * result
38. + ((userName ==null) ?0: userName.hashCode());
39. returnresult;
40. }
42. @Override
43. publicbooleanequals(Object obj) {
44. if(this== obj)
45. returntrue;
46. if(obj ==null)
47. returnfalse;
48. if(getClass() != obj.getClass())
49. returnfalse;
50. UserId other = (UserId) obj;
51. if(userCode ==null) {
52. if(other.userCode !=null)
53. returnfalse;
54. }elseif(!userCode.equals(other.userCode))
55. returnfalse;
56. if(userName ==null) {
57. if(other.userName !=null)
58. returnfalse;
59. }elseif(!userName.equals(other.userName))
60. returnfalse;
61. returntrue;
62. }
64. @Override
65. publicString toString() {
66. return"UserId [userCode="+ userCode +", userName="+ userName +"]";
67. }
70. }

User:

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1. publicclassUserimplementsSerializable {
2. privateUserId userId;
3. privateString password;

查询比较特别：

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1. UserId userId =newUserId("code","hello");
2. User user = (User) session.get(User.class, userId);

3、复合主键中的外键/4、复合主键的外键
代码：

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1. publicclassUserimplementsSerializable {
2. privateUserId userId;
3. privateString password;
4. privateDepartment department;
5. privateSet<Item> items =newHashSet<Item>();

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1. publicclassDepartmentimplementsSerializable {
2. privateInteger departmentId;
3. privateString departmentName;
4. privateSet<User> users =newHashSet<User>();

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1. publicclassItemimplementsSerializable {
2. privateInteger itemId;
3. privateString itemName;
4. privateUser user;

配置文件：

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1. <classname="User"table="USER">
2. <composite-id name="userId"class="UserId">
3. <key-property name="userCode"column="USER_CODE"></key-property>
4. <key-property name="userName"column="USER_NAME"></key-property>
5. </composite-id>
6. <property name="password"></property>
8. <many-to-one name="department"class="Department"
9. column="DEPARTMENT_ID"
10. cascade="save-update"></many-to-one>
12. <set name="items"cascade="save-update"inverse="true">
13. <key>
14. <column name="USER_CODE"></column>
15. <column name="USER_NAME"></column>
16. </key>
17. <one-to-manyclass="Item"/>
18. </set>
19. </class>

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1. <classname="Department"table="Department">
2. <id name="departmentId"column="DEPARTMENT_ID">
3. <generatorclass="native"></generator>
4. </id>
5. <property name="departmentName"column="department_Name"></property>
7. <set name="users"inverse="true"cascade="save-update">
8. <key column="DEPARTMENT_ID"></key>
9. <one-to-manyclass="User"/>
10. </set>
11. </class>

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1. <classname="Item"table="Item">
2. <id name="itemId"column="item_Id">
3. <generatorclass="native"></generator>
4. </id>
5. <property name="itemName"column="item_Name"></property>
7. <many-to-one name="user"class="User">
8. <column name="USER_CODE"></column>
9. <column name="USER_NAME"></column>
10. </many-to-one>
11. </class>

插入：

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1. User user =newUser("123");
2. UserId userId =newUserId("code","hello");
3. user.setUserId(userId);
5. Department department =newDepartment("de hello");
6. user.setDepartment(department);
8. Item item =newItem("item1");
9. user.getItems().add(item);
11. //必须添加，因为，在<set name="items" cascade="save-update" inverse="true">设置了反向控制，
12. //那么就由Item的user属性来控制之间的关系，所以Item的user必须知道user的存在
13. //要知道这个东西和cascade="save-update"并不想干，cascade只是负责是否级联保存的，和表之间的关系由谁维护无关(这一点相信很多人都会搞混)
14. item.setUser(user);
16. session.save(user);

5、利用注解的复合键

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1. @Embeddable
2. publicclassUserIdimplementsSerializable {
3. privateString userCode;
4. privateString userName;

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1. @Entity
2. publicclassUserimplementsSerializable {
3. @Id
4. @AttributeOverrides({
5. @AttributeOverride(name="userCode",
6. column=@Column(name="user_Code")),
7. @AttributeOverride(name="userName",
8. column=@Column(name="user_Name"))
10. })
11. privateUserId userId;
12. privateString password;
13. @OneToMany(mappedBy="user")
14. privateSet<Item> items =newHashSet<Item>();

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1. @Entity
2. publicclassItemimplementsSerializable {
3. @Id
4. @GeneratedValue
5. @Column(name="ITEM_ID")
6. privateInteger itemId;
7. privateString itemName;
8. //KEY `FK22EF338367E49C` (`user_Code`,`user_Name`),
9. //CONSTRAINT `FK22EF338367E49C` FOREIGN KEY (`user_Code`, `user_Name`) REFERENCES `user` (`user_Code`, `user_Name`)
10. @ManyToOne
11. @JoinColumns({
12. @JoinColumn(name="user_Code"),
13. @JoinColumn(name="user_Name")
14. })
15. privateUser user;