JPA一对多和多对一关系

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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

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：

重点在于：

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：