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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來鍵
- class（可選）： 設定one方對應的持久化類的名稱，即持久化類屬性的型別
- 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來鍵約束
- cascade（可選）： 級聯操作選項，預設為none

單向多對一(@ManyToOne)關聯是最常見的單向關聯關係。假設多種商品(Product)可以有一個商品型別(ProductType),只關心商品(Product)實體找到對應的商品型別(ProductType)實體，而無須關心從某個商品型別(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 [email protected]
 * @description:單向：多個產品屬於同一個產品型別
 */
@Entity
@Table(name = "t_product")
public class Product {
  @Id
  @GeneratedValue
  private Long id;
  private String name;
  // 多對一：optional=false表示外來鍵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;
  }

  @Override
  public 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 [email protected]
 * @description:單向
 */
@Entity
@Table(name = "t_product_type")
public class ProductType {
  @Id
  @GeneratedValue
  private 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;
  }

  @Override
  public 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 [email protected]
 * @description:單向多對一Test
 */
public class Many2OneTest {
  private static EntityManagerFactory entityManagerFactory = null;
  private EntityManager entityManager = null;

  @BeforeClass
  public static void setUpBeforeClass() throws Exception {
    entityManagerFactory = Persistence.createEntityManagerFactory("com.jege.jpa");
  }

  // 要求：一次性儲存1個產品型別，儲存2個產品
  // 單向多對一儲存的時候必須先儲存一方，否則會出現多餘的update語句，從而影響效能
  @Before
  public void persist() throws Exception {
    entityManager = entityManagerFactory.createEntityManager();
    entityManager.getTransaction().begin();

    ProductType type = new ProductType();
    type.setName("產品型別1");

    // 傳入type的本質是處理資料庫product表的type_id外來鍵
    Product product1 = new Product("產品1", null);
    Product product2 = new Product("產品2", type);

    System.out.println("儲存之前：" + type);
    entityManager.persist(type);// JPA會自動把儲存後的主鍵放到當前物件的id裡面
    System.out.println("儲存之後：" + 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，來判斷是否有產品型別
  @Test
  public 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型別的");
    } else {
      System.out.println("當前產品是有產品型別的");
    }
  }

  @After
  public void tearDown() throws Exception {
    entityManager.getTransaction().commit();
    if (entityManager != null && entityManager.isOpen())
      entityManager.close();
  }

  @AfterClass
  public static void tearDownAfterClass() throws Exception {
    if (entityManagerFactory != null && entityManagerFactory.isOpen())
      entityManagerFactory.close();
  }

}

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