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我有一个关于在 EJB 中设计 ManyToMany 的问题，可连接对象如何具有属性？
这里举个例子，学生和课程是ManyToMany，每个学生都有很多课程，很多学生选择一门课程。

    @Entity
    public class Student implements Serializable { 
        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        Long id;
        String name;
        private Collection<Course> courses; 

        @ManyToMany(mappedBy = "students",cascade=CascadeType.ALL)      
        public Collection<Course> getCourses() {
            return this.courses;
        }

        public void setCourses(Collection<Course> courses) {
            this.courses = courses;
        }

    }


    @Entity
    public class Course implements Serializable { 
        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        Long id;
        String name; 
        private Collection<Student> students; 

        @ManyToMany(cascade=CascadeType.ALL)
        @JoinTable(name = "Student_Course",
        joinColumns = {@JoinColumn(name = "Course_ID", referencedColumnName = "id")}, 
        inverseJoinColumns = {@JoinColumn(name = "Student_ID", referencedColumnName = "id")})  

        public Collection<Student> getStudents() {
            return this.students;
        }

        public void setStudents(Collection<Student> students) {
            this.students = students;
        }
    }

但是，如果我在 JoinTable 中有一个属性，例如每个学生有一门课程的分数。如何使用 ManyToMany 在 EJB 中实现它？
非常感谢您的关注！

4

3 回答 3

4

这是不可能的，您不能将属性添加到关系中。如果您需要访问连接表中的属性，则该属性属于某个实体，因此您需要第三个实体。

于 2014-11-12T09:42:58.713 回答
3

有可能的。

您只需要通过第三个实体将多对多映射替换为一对多和多对一映射的显式组合，这将表示两个主要实体之间的关联（示例中的学生和课程） ）。

请在此处阅读详细信息

于 2015-06-10T15:54:41.227 回答
0

具有多对多关系的实体（商家和服务）。这可以使用以下第三方实体来实现：-

@Entity
@Table(name = "merchant")
public class Merchant implements java.io.Serializable {

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.merchant",targetEntity = MerchantService.class)
    private Set<MerchantService> merchantServices = new HashSet<>();
}

@Entity
@Table(name = "merchant_service")
@AssociationOverrides({
        @AssociationOverride(name = "pk.merchant",
            joinColumns = @JoinColumn(name = "merchant_id")),
        @AssociationOverride(name = "pk.service",
            joinColumns = @JoinColumn(name = "service_id")) })
public class MerchantService implements java.io.Serializable {

    @EmbeddedId
    private MerchantServiceId pk = new MerchantServiceId();

    private boolean isActive;

    public MerchantServiceId getPk() {
        return pk;
    }

    public void setPk(MerchantServiceId pk) {
        this.pk = pk;
    }

    @Transient
    public Service getService() {
        return getPk().getService();
    }


    @Transient
    public Merchant getMerchant() {
        return getPk().getMerchant();
    }


    public boolean isActive() {
        return isActive;
    }

    public void setActive(boolean isActive) {
        this.isActive = isActive;
    }

}

@Embeddable
public class MerchantServiceId implements java.io.Serializable {

    private Merchant merchant;
    private Service service;

    @ManyToOne
    public Merchant getMerchant() {
        return merchant;
    }

    @ManyToOne
    public Service getService() {
        return service;
    }

}

@Entity
@Table(name = "service")
public class Service implements java.io.Serializable {

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.service",targetEntity = MerchantService.class)
    private Set<MerchantService> merchantServices = new HashSet<>();

}
于 2016-11-08T12:17:45.870 回答