多对一双向关联

ps：双向关联与单向关联的区别在于

1、双向关联：A能推出B，B也能推出A。

　　单向关联，只能由A推出B或者是B推出A。

例如：一个小组有多个员工，一个员工对应一个组。从员工的角度说是多对一，从小组的角度是一对多。

package com.model;

import java.util.Set;

import javax.persistence.Entity;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
@Table(name="group")
public class Group {
    private Integer id;
    private String groupName;
    private Set<Worker> workers;
    
    @OneToMany(mappedBy="group")
    public Set<Worker> getWorkers() {
        return workers;
    }
    public void setWorkers(Set<Worker> workers) {
        this.workers = workers;
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getGroupName() {
        return groupName;
    }
    public void setGroupName(String groupName) {
        this.groupName = groupName;
    }
}

package com.model;

import java.io.Serializable;
import java.util.Date;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Entity
@Table(name="worker")
public class Worker implements Serializable{
    /**
     * @author yzy
     * @2014年11月1日15:31:08
     */
    private static final long serialVersionUID = 1L;
    private Integer id;
    private String name;
    private String password;
    private Integer sex;
    private Integer age;
    private Date birthdate;
    private Group group;
    private String position;
    private String telephone;
    private Date enterDate;
    private Float performance;
    private Integer gold;
    
    public Worker(){
        
    }
    
    @Id @GeneratedValue(strategy=GenerationType.AUTO)
    @Column(name="worker_id")
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    @Column(name="worker_name")
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @Column(name="worker_password")
    public String getPassword() {
        return password;
    }
    public void setPassword(String password) {
        this.password = password;
    }
    @Column(name="worker_sex")
    public Integer getSex() {
        return sex;
    }
    public void setSex(Integer sex) {
        this.sex = sex;
    }
    @Column(name="worker_age")
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
    @Column(name="worker_birthdate")
    public Date getBirthdate() {
        return birthdate;
    }
    public void setBirthdate(Date birthdate) {
        this.birthdate = birthdate;
    }
    @ManyToOne//多对一，一个员工只能在一个小组，一个小组有多名成员
    @JoinColumn(name="group_id")
    public Group getGroup() {
        return group;
    }
    public void setGroup(Group group) {
        this.group = group;
    }
    @Column(name="worker_position")
    public String getPosition() {
        return position;
    }
    public void setPosition(String position) {
        this.position = position;
    }
    @Column(name="worker_telephone")
    public String getTelephone() {
        return telephone;
    }
    public void setTelephone(String telephone) {
        this.telephone = telephone;
    }
    @Column(name="worker_enterDate")
    public Date getEnterDate() {
        return enterDate;
    }
    public void setEnterDate(Date enterDate) {
        this.enterDate = enterDate;
    }
    @Column(name="worker_performance")
    public Float getPerformance() {
        return performance;
    }
    public void setPerformance(Float performance) {
        this.performance = performance;
    }
    @Column(name="worker_gold")
    public Integer getGold() {
        return gold;
    }
    public void setGold(Integer gold) {
        this.gold = gold;
    }
    
    
}

一对多时在多的那一方设置一的外键，如上是在Worker中设置Group的外键。双向关联必设 mappedBy属性，总是由多的一方来维护。
举例：员工和组 一个group可以有多个worker，这里从group来说是一对多 而多个worker可以属于一个group 这里从worker来说是多对一 所以
不管是多对一还是一对多 都是在多的那方加外键。也就是说双向关联时在一的那一方设置mappedBy，即在Group实体中的getWorkers()方法上头加@OneToMany(mappedBy="group")