如何使用JPA创建多对一映射？

我希望实现以下模式：

表A:

a\u id

（其他栏目）

---

表B:

b_id

（其他栏目）

---

表C:

c\u id

（其他栏目）

---

表D:

a\u id\u fk

b\u id\u fk

c\u id\u fk

我想知道如何创建实体D。表D中的所有键

a_id_fk

、

b_id_fk

和

c_id_fk

构成一个复合主键

---

谢谢

它模拟了三元关联。为了使用它，你需要一张地图。JPA1.0不支持Map，所以如果您想使用它，您需要Hibernate，因为它的Map支持。有点像

@Entity
public class A {

    @ManyToMany
    @org.hibernate.annotations.MapKeyManyToMany(
        joinColumns=@JoinColumn(name="B_ID")
    )
    @JoinTable(
        name="D",
        joinColumns=@JoinColumn(name="A_ID"),
        inverseJoinColumns=@JoinColumn(name="C_ID")
    )
    private Map<B, C> bAndC = new HashMap<B, C>();

}

或者，如果不需要地图，可以根据ABC对实体建模

public class AbC {

    @ManyToOne
    @JoinColumn(name="A_ID", insertable=false, updateable=false)
    private A a;

    @ManyToOne
    @JoinColumn(name="B_ID", insertable=false, updateable=false)
    private B b;

    @ManyToOne
    @JoinColumn(name="C_ID", insertable=false, updateable=false)
    private C c;

    @EmbeddedId
    private A_b_C_i_D id;

    @Embeddable
    public static class A_b_C_i_D implements Serializable {

        @Column(name="A_ID", updateable=false)
        private Integer a_i_d;

        @Column(name="B_ID", updateable=false)
        private Integer b_i_d;

        @Column(name="C_ID", updateable=false)
        private Integer c_i_d;

        // getter's and setter's

        public boolean equals(Object o) {
            if(o == null)
                return false;

            if(!(o instanceof A_b_C_i_D))
                return false;

            A_b_C_i_D other = (A_b_C_i_D) o;
            if(!(getA_i_d().equals(other.getA_i_d()))
                return false;

            if(!(getB_i_d().equals(other.getB_i_d()))
                return false;

            if(!(getC_i_d().equals(other.getC_i_d()))
                return false;

            return true;
        }

        public int hashcode() {
            // hashcode implementation
        }

    }

}

---

问候，

谢谢。非常具有描述性和帮助性

public class AbC {

    @ManyToOne
    @JoinColumn(name="A_ID", insertable=false, updateable=false)
    private A a;

    @ManyToOne
    @JoinColumn(name="B_ID", insertable=false, updateable=false)
    private B b;

    @ManyToOne
    @JoinColumn(name="C_ID", insertable=false, updateable=false)
    private C c;

    @EmbeddedId
    private A_b_C_i_D id;

    @Embeddable
    public static class A_b_C_i_D implements Serializable {

        @Column(name="A_ID", updateable=false)
        private Integer a_i_d;

        @Column(name="B_ID", updateable=false)
        private Integer b_i_d;

        @Column(name="C_ID", updateable=false)
        private Integer c_i_d;

        // getter's and setter's

        public boolean equals(Object o) {
            if(o == null)
                return false;

            if(!(o instanceof A_b_C_i_D))
                return false;

            A_b_C_i_D other = (A_b_C_i_D) o;
            if(!(getA_i_d().equals(other.getA_i_d()))
                return false;

            if(!(getB_i_d().equals(other.getB_i_d()))
                return false;

            if(!(getC_i_d().equals(other.getC_i_d()))
                return false;

            return true;
        }

        public int hashcode() {
            // hashcode implementation
        }

    }

}