hibernate多對多對映拆成2個一對多對映（註解）
hibernate的many to many確實很是方便我們處理實體和集合間的關係，並可以通過級聯的方法處理集合，但有的時候many to many不能滿足我們的需要，比如 使用者<--->選課，典型的多對多關係，一般情況下，會生成
course_user(course_id,user_id);
但使用者選課的時候最好加入稽核功能，所以我們希望在中間自動生成的表中加入一個boolean欄位，類似這種結構:
course_user(course_id,user_id,accessable);
這個時候我們可以把many to many拆分成2個many to one ，自己手動添加個中間表，這樣擴充套件就好很多了。
程式碼如下：
@Embeddable
public class CourseUserPK implements Serializable{
private static final long serialVersionUID = 1L;
private Course course;
private User user;
@ManyToOne
@JoinColumn(name = "course_id", nullable = false)
public Course getCourse() {
return course;
}
public void setCourse(Course course) {
this.course = course;
}
@ManyToOne
@JoinColumn(name = "user_id", nullable = false)
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
public boolean equals(Object object) {
if (this == object)
return true;
if (!(object instanceof CourseUserPK))
return false;
final CourseUserPK other = (CourseUserPK) object;
if (this.course != null && other.getCourse() != null) {
if(course.getId() == other.course.getId()) {
if(user!=null&&other.getUser()!=null&&user.getId()==other.getUser().getId()) {
return true;
} else {
return false;
}
} else {
return true;
}
} else
return false;
}
public int hashCode() {
return super.hashCode()+
(course!=null?course.hashCode():0)+
(user!=null?user.hashCode():0);
}
}
@Entity
@IdClass(CourseUserPK.class)
public class CourseTeacher {
private Course course;
private User user;
private Boolean accessable;//true user is in course, otherwise not
public CourseTeacher(Course course, User user, Boolean accessable) {
this.course = course;
this.user = user;
this.accessable = accessable;
}
@Id
public Course getCourse() {
return course;
}
public void setCourse(Course course) {
this.course = course;
}
@Id
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
public Boolean getAccessable() {
return accessable;
}
public void setAccessable(Boolean accessable) {
this.accessable = accessable;
}
}
@Entity
@IdClass(CourseUserPK.class)
public class CourseStudent {
private Course course;
private User user;
private Boolean accessable;//true user is in course, otherwise not
public CourseStudent(Course course, User user, Boolean accessable) {
this.course = course;
this.user = user;
this.accessable = accessable;
}
@Id
public Course getCourse() {
return course;
}
public void setCourse(Course course) {
this.course = course;
}
@Id
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
public Boolean getAccessable() {
return accessable;
}
public void setAccessable(Boolean accessable) {
this.accessable = accessable;
}
}
@Entity
public class Course extends BaseEntity {
private static final long serialVersionUID = 8768227695335084711L;
private Set<CourseTeacher> teachers;
private Set<CourseStudent> students;
@OneToMany(mappedBy="course",cascade=CascadeType.ALL)
public Set<CourseTeacher> getTeachers() {
return teachers;
}
public void setTeachers(Set<CourseTeacher> teachers) {
this.teachers = teachers;
}
@OneToMany(mappedBy="course")
public Set<CourseStudent> getStudents() {
return students;
}
public void setStudents(Set<CourseStudent> students) {
this.students = students;
}
}
Hibernate可以通過*.hbm.xml配置檔案能很好地把多對多關係拆分成兩個一對多的關係，但Hibernate Annotation的文件中沒有說到這個點上來。下面通過例項說明用註解來實現多對多拆分成兩個一對多。
下面以商品Product和訂單Order之間的多對多關係來說明。
Product的屬性包括:
- id
- name
- price
Order的屬性包括:
- id
- date(下訂單的時間)
為什麼要把多對多關係拆分成兩個一對多?
因為多對多關係不能儲存兩個實體之間共有的屬性。比如，如何記錄訂單A中購買的商品B的數量呢？如果以多對多對映就不能實現了。
中間實體----用來記錄兩個多對多實體之間共有關係
在Product和Order之間的關係中，可以用一個OrderItem實體來表示兩者多對多中的眾多關係中的一個關係。即Product與OrderItem，Order與OrderItem之間的關係為一對多的關係。
OrderItem的屬性包括:
- product(假如當前記錄的是商品C)
- order(假如當前記錄的是訂單D)
- quantity(這裡記錄的是訂單D中商品C的數量)
例項程式碼（省略了類包引用）:
複合主鍵類:
@Embeddable
public class OrderItemPK implements Serializable{
private Product product;
private Order order;
@ManyToOne
@JoinColumn(name="product_id",referencedColumnName="id")
public Product getProduct(){
return product;
}
public void setProduct(Product product){
this.product=product;
}
@ManyToOne
@JoinColumn(name="order_id",referencedColumnName="id")
public Order getOrder(){
return order;
}
public void setOrder(Order order){
this.order=order;
}
public boolean equals(Object object){...}
public int hashCode(){...}
}
OrderItem類:
@Entity
@org.hibernate.annotation.Entity(dynamicInsert=true,dynamicUpdate=true)
@Table(name="order_item")
@IdClass(OrderItemPK.class)
public class OrderItem implements Serializable{
private Product product;
private Order order;
private int quantity;
@Id
public Product getProduct(){
return product;
}
public void setProduct(Product product){
this.product=product;
}
@Id
public Order getOrder(){
return order;
}
public void setOrder(Order order){
this.order=order;
}
@Column(name="quantity")
public int getQuantity(){
return quantity;
}
public void setQuantity(int quantity){
this.quantity=quantity;
}
}
Product類:
@Entity
@org.hibernate.annotation.Entity(dynamicInsert=true,dynamicUpdate=true)
@Table(name="product")
public class Product implements Serializable{
private int id;
private String name;
private double price;
private Set<OrderItem> orderItems;
@Id
@GenericGenerator(name="g_id",strategy="increment")
@GeneratedValue(generator="g_id")
public int getId(){
return id;
}
public void setId(int id){
this.id=id;
}
public String getName(){
return name;
}
public void setName(String name){
this.name=name;
}
public double getPrice(){
return price;
}
public void setPrice(double price){
this.price=price;
}
@OneToMany(mappedBy="product")
public Set<OrderItem> getOrderItems(){
return orderItems;
}
public void setOrderItems(Set<OrderItem> orderItems){
this.orderItems=orderItems;
}
}
Order類:
@Entity
@org.hibernate.annotation.Entity(dynamicInsert=true,dynamicUpdate=true)
@Table(name="tbl_order")
public class Order implements Serializable{
private int id;
private Calendar date;
private Set<OrderItem> orderItems;
@Id
@GenericGenerator(name="g_id",strategy="increment")
@GeneratedValue(generator="g_id")
public int getId(){
return id;
}
public void setId(int id){
this.id=id;
}
public Calendar getDate(){
return date;
}
public void setDate(Calendar date){
this.date=date;
}
@OneToMany(mappedBy="order")
public Set<OrderItem> getOrderItems(){
return orderItems;
}
public void setOrderItems(Set<OrderItem> orderItems){
this.orderItems=orderItems;
}
}