Hibernate One class to two table or Two class to one table

利用Hibernate可以很方便地将对象持久化。但有时候，我们想要将一个class持久化成2张表，这往往是为了数据库的范式需求。

而有时候，我们可能希望将2个class持久化到一张表中去。

利用Hibernate可以很方便的实现。

1. One class to two table

比如我们有一个Cusutomer 类，有以下这些属性，我们想要让customerId, customerName放到一张表中去，其他的属性放到CustomerDetail表中去。

private int customerId;private String customerName;private String customerAddress;private int creditScore;private int rewardPoints;

为了实现这个，我们需要告诉hibernate我们要创建哪两张表和class属性分别属于哪个表。

package com.hibernate.chapter2;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.SecondaryTable;import javax.persistence.Table;@Entity@Table(name="Customer")@SecondaryTable(name="CustomerInfo")public class Customer {private int customerId;private String customerName;private String customerAddress;private int creditScore;private int rewardPoints;@Id@GeneratedValuepublic int getCustomerId() {return customerId;}public void setCustomerId(int customerId) {this.customerId = customerId;}public String getCustomerName() {return customerName;}public void setCustomerName(String customerName) {this.customerName = customerName;}@Column(table="CustomerInfo")public String getCustomerAddress() {return customerAddress;}public void setCustomerAddress(String customerAddress) {this.customerAddress = customerAddress;}@Column(table="CustomerInfo")public int getCreditScore() {return creditScore;}public void setCreditScore(int creditScore) {this.creditScore = creditScore;}@Column(table="CustomerInfo")public int getRewardPoints() {return rewardPoints;}public void setRewardPoints(int rewardPoints) {this.rewardPoints = rewardPoints;}}

在上面的代码中我们要生成2个表，Customer 和 CustomerInfo。同时指明了某些column要放到CustomerInfo表中。不指明，就是默认放到第一张表中去。

最后我们看生成的表的情况：Customer的主键是customerId，CustomerInfo的外键是Customer表中的customerId。

2. Two class to one table

比如我们有2个class：

School:

private int schoolId;private String schoolName;

SchoolDetail

private String  schoolAddress;private boolean isPublicSchool;private int studentCount;

我们想要将这两个class持久化到同一张表中去。

我们先想想，如果没有hibernate的话，我们会怎么实现。

在创建表的时候，我们想把2个class中属性都作为table的column。

在insert record的时候，我们会将2个class中的属性值都放在一起insert。

hibernate的实现也是类似，它是将一个class当做主class，将另一个class相放到这个主class中去(embed)。

主class School.java代码：

package com.hibernate.chapter3;import javax.persistence.Embedded;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;@Entitypublic class School {private int schoolId;private String schoolName;@Embeddedprivate SchoolDetail schoolDetail;@Id@GeneratedValuepublic int getSchoolId() {return schoolId;}public void setSchoolId(int schoolId) {this.schoolId = schoolId;}public String getSchoolName() {return schoolName;}public void setSchoolName(String schoolName) {this.schoolName = schoolName;}public SchoolDetail getSchoolDetail() {return schoolDetail;}public void setSchoolDetail(SchoolDetail schoolDetail) {this.schoolDetail = schoolDetail;}}

嵌入的class SchoolDetail.java

package com.hibernate.chapter3;import javax.persistence.Embeddable;@Embeddablepublic class SchoolDetail {private String  schoolAddress;private boolean isPublicSchool;private int studentCount;public String getSchoolAddress() {return schoolAddress;}public void setSchoolAddress(String schoolAddress) {this.schoolAddress = schoolAddress;}public int getStudentCount() {return studentCount;}public void setStudentCount(int studentCount) {this.studentCount = studentCount;}public boolean isPublicSchool() {return isPublicSchool;}public void setPublicSchool(boolean isPublicSchool) {this.isPublicSchool = isPublicSchool;}}

insert record 代码：

package com.hibernate.chapter3;import org.hibernate.Session;import org.hibernate.SessionFactory;import org.hibernate.cfg.AnnotationConfiguration;import org.hibernate.tool.hbm2ddl.SchemaExport;public class TestSchool {public static void main(String[] args) {AnnotationConfiguration config = new AnnotationConfiguration();config.addAnnotatedClass(School.class);// ask hibernate to persistent this classconfig.configure();//default will read hibernate.cfg.xml new SchemaExport(config).create(true, true);//create table, first Boolean means print sql to the log file, second Boolean means execute sql//insert recordSessionFactory factory = config.buildSessionFactory();Session session = factory.getCurrentSession();session.beginTransaction();SchoolDetail schoolDetial = new SchoolDetail();schoolDetial.setSchoolAddress("Shanghai");schoolDetial.setPublicSchool(true);schoolDetial.setStudentCount(500);School school = new School();school.setSchoolName("school name");school.setSchoolDetail(schoolDetial);session.save(school);//associate alex instance with hibernatesession.getTransaction().commit();//will execute sql}}