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")
	public 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();
	}
}