JPA中的@ManyToOne

主要屬性
- name(必需): 設定“many”方所包含的“one”方所對應的持久化類的屬性名
- column（可選）： 設定one方的主鍵，即持久化類的屬性對應的表的外鍵
- class（可選）： 設定one方對應的持久化類的名稱，即持久化類屬性的類型
- not-null（可選）： 如果為true,，表示需要建立相互關聯的兩個表之間的外鍵約束
- cascade（可選）： 級聯操作選項，默認為none

單向多對一(@ManyToOne)關聯是最常見的單向關聯關系。假設多種商品(Product)可以有一個商品類型(ProductType),只關心商品(Product)實體找到對應的商品類型(ProductType)實體，而無須關心從某個商品類型(ProductType)找到全部商品(Product).

單向多對一表的ddl語句

CREATE TABLE `t_product_type` (`id` bigint(20) NOT NULL AUTO_INCREMENT,`name` varchar(255) DEFAULT NULL,PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;CREATE TABLE `t_product` (`id` bigint(20) NOT NULL AUTO_INCREMENT,`name` varchar(255) DEFAULT NULL,`type_id` bigint(20) DEFAULT NULL,PRIMARY KEY (`id`),KEY `FK_oyt6r2g6hwbyee5adel4yj59e` (`type_id`),CONSTRAINT `FK_oyt6r2g6hwbyee5adel4yj59e` FOREIGN KEY (`type_id`) REFERENCES `t_product_type` (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

Product

package com.jege.jpa.many2one;import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.JoinColumn; import javax.persistence.ManyToOne; import javax.persistence.Table;/*** @author JE哥* @email 1272434821@qq.com* @description:單向：多個產品屬于同一個產品類型*/ @Entity @Table(name = "t_product") public class Product {@Id@GeneratedValueprivate Long id;private String name;// 多對一：optional=false表示外鍵type_id不能為空@ManyToOne(optional = true)@JoinColumn(name = "type_id")private ProductType type;public Product() {}public Product(String name, ProductType type) {this.name = name;this.type = type;}public Long getId() {return id;}public void setId(Long id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public ProductType getType() {return type;}public void setType(ProductType type) {this.type = type;}@Overridepublic String toString() {return "Product [id=" + id + ", name=" + name + "]";}}

ProductType

package com.jege.jpa.many2one;import javax.persistence.Entity; import javax.persistence.GeneratedValue; import javax.persistence.Id; import javax.persistence.Table;/*** @author JE哥* @email 1272434821@qq.com* @description:單向*/ @Entity @Table(name = "t_product_type") public class ProductType {@Id@GeneratedValueprivate Long id;private String name;public Long getId() {return id;}public void setId(Long id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}@Overridepublic String toString() {return "ProductType [id=" + id + ", name=" + name + "]";}}

Many2OneTest

package com.jege.jpa.many2one;import javax.persistence.EntityManager; import javax.persistence.EntityManagerFactory; import javax.persistence.Persistence;import org.junit.After; import org.junit.AfterClass; import org.junit.Before; import org.junit.BeforeClass; import org.junit.Test; /*** @author JE哥* @email 1272434821@qq.com* @description:單向多對一Test*/ public class Many2OneTest {private static EntityManagerFactory entityManagerFactory = null;private EntityManager entityManager = null;@BeforeClasspublic static void setUpBeforeClass() throws Exception {entityManagerFactory = Persistence.createEntityManagerFactory("com.jege.jpa");}// 要求：一次性保存1個產品類型，保存2個產品// 單向多對一保存的時候必須先保存一方，否則會出現多余的update語句，從而影響性能@Beforepublic void persist() throws Exception {entityManager = entityManagerFactory.createEntityManager();entityManager.getTransaction().begin();ProductType type = new ProductType();type.setName("產品類型1");// 傳入type的本質是處理數據庫product表的type_id外鍵Product product1 = new Product("產品1", null);Product product2 = new Product("產品2", type);System.out.println("保存之前：" + type);entityManager.persist(type);// JPA會自動把保存后的主鍵放到當前對象的id里面System.out.println("保存之后：" + type);entityManager.persist(product1);entityManager.persist(product2);System.out.println("++++++++++++++++++++");}// 可以通過多方Product獲取一方ProductType是否為null，來判斷是否有產品類型@Testpublic void find() throws Exception {Product product = entityManager.find(Product.class, 2L);System.out.println(product);ProductType type = product.getType();if (type == null) {System.out.println("當前產品是沒有產品類型的");} else {System.out.println("當前產品是有產品類型的");}}@Afterpublic void tearDown() throws Exception {entityManager.getTransaction().commit();if (entityManager != null && entityManager.isOpen())entityManager.close();}@AfterClasspublic static void tearDownAfterClass() throws Exception {if (entityManagerFactory != null && entityManagerFactory.isOpen())entityManagerFactory.close();}}

其他關聯項目

• JPA 系列教程2-單表操作
  http://blog.csdn.net/je_ge/article/details/53493352

源碼地址

https://github.com/je-ge/jpa

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轉載于:https://www.cnblogs.com/je-ge/p/6152949.html

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