JPA一对多和多对一关系

 1-m:多的一方为关系维护端,关系维护端负责外键纪录的更新,关系被维护端没有权力更新外键纪录.

维护端注解

@OneToMany(cascade = { CascadeType.PERSIST, CascadeType.REFRESH,
            CascadeType.MERGE, CascadeType.REMOVE },
                       fetch = FetchType.EAGER,
                       mappedBy = "order")

被维护端注解

@ManyToOne(cascade = { CascadeType.MERGE, CascadeType.REFRESH }, optional = false)
@JoinColumn(name = "order_id")

对于一对多和多对一关系的现实中的例子是，网上购物时的订单和订单项。一个订单有多个订单项。多个订单项属于一个订单。

订单实体类：

package com.taoistwar.jpa.entity.onetomany;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
@Table(name = "order_info")
public class OrderInfo {
    private Integer id;
    private String name;

    private Set<OrderItem> items = new HashSet<OrderItem>();

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @OneToMany(cascade = { CascadeType.PERSIST, CascadeType.REFRESH,
            CascadeType.MERGE, CascadeType.REMOVE }, fetch = FetchType.EAGER,
                       mappedBy = "order")
    public Set<OrderItem> getItems() {
        return items;
    }

    public void setItems(Set<OrderItem> items) {
        this.items = items;
    }

    public void addOrderItem(OrderItem orderItem) {
        orderItem.setOrder(this);
        this.items.add(orderItem);
    }

}

 重点在于：

     @OneToMany(cascade = { CascadeType.PERSIST, CascadeType.REFRESH,
    CascadeType.MERGE, CascadeType.REMOVE }, fetch = FetchType.EAGER,
                     mappedBy = "order")
ublic Set<OrderItem> getItems() {
return items;

 订单项实体类：

package com.taoistwar.jpa.entity.onetomany;

import javax.persistence.CascadeType;
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 = "order_item")
public class OrderItem {
    private Integer Id;
    private String name;

    private OrderInfo order;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Integer getId() {
        return Id;
    }

    public void setId(Integer id) {
        Id = id;
    }

    @Column(length = 20, nullable = true)
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @ManyToOne(cascade = { CascadeType.MERGE, CascadeType.REFRESH }, optional = false)
    @JoinColumn(name = "order_id")
    public OrderInfo getOrder() {
        return order;
    }

    public void setOrder(OrderInfo order) {
        this.order = order;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((Id == null) ? 0 : Id.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        OrderItem other = (OrderItem) obj;
        if (Id == null) {
            if (other.Id != null)
                return false;
        } else if (!Id.equals(other.Id))
            return false;
        return true;
    }

}

 重点在于：

      @ManyToOne(cascade = { CascadeType.MERGE, CascadeType.REFRESH }, optional = false)
@JoinColumn(name = "order_id")
public OrderInfo getOrder() {
    return order;
}

 测试类：

package com.taoistwar.jpa.entity.onetomany;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

import org.junit.Test;


public class OneToMany {
    @Test
    public void save() {
        EntityManagerFactory emf = Persistence
                .createEntityManagerFactory("JPAPU");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        OrderInfo o = new OrderInfo();
        o.setName("订单一");
        OrderItem oi1 = new OrderItem();
        oi1.setName("产品一");
        o.addOrderItem(oi1);
        OrderItem oi2 = new OrderItem();
        oi2.setName("产品一");
        o.addOrderItem(oi2);
        OrderItem oi3 = new OrderItem();
        oi3.setName("产品一");
        o.addOrderItem(oi3);
        OrderItem oi4 = new OrderItem();
        oi4.setName("产品一");
        o.addOrderItem(oi4);
        em.persist(o);
        // UUID.randomUUID().toString();
        em.getTransaction().commit();
        emf.close();
    }
}

 

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