Java JPA ORM OneToMany ManyToOne

Question

我试图定义两个类之间的双向关系。拥有方是类测验，相反方是用户。一个用户可以创建许多测验，而一个测验只能有一个创建它的用户。我在网上找到的每一个教程都指出，在owning方面，您指定了ManyToOne注释和JoinColumn，在相反的方面，您使用owners字段的名称指定了OneToMany和mappedBy。然而，当我这样做时，IDE给了我一个错误“找不到逆关系”。我在这个概念上哪里出错了？如有任何帮助或指导，将不胜感激。

Build.Gradle

plugins {
    id 'org.springframework.boot' version '2.2.2.RELEASE'
    id 'java'
    id "io.freefair.lombok" version "6.1.0-m3"
}

apply plugin: 'io.spring.dependency-management'

sourceCompatibility = 11

repositories {
    mavenCentral()
}

sourceSets.main.resources.srcDirs = ["src/resources"]

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter:2.5.3'
    implementation 'org.springframework.boot:spring-boot-starter-actuator:2.5.3'
    implementation 'org.springframework.boot:spring-boot-starter-web:2.5.3'
    implementation 'org.springframework.boot:spring-boot-starter-data-jpa:2.5.3'
    implementation 'org.springframework.boot:spring-boot-starter-security:2.5.3'
    implementation 'org.springframework.boot:spring-boot-starter-validation:2.5.3'
    runtimeOnly 'com.h2database:h2:1.4.200'
}

Quiz.java

@Entity
@Table(name = "quizzes")
public class Quiz {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;

    ...

    @ManyToOne
    @JoinColumn(name = "UserID")
    private User createdBy;

}

@Entity
@Table(name = "users")
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    ...

    @OneToMany(mappedBy = "createdBy")
    @MapKey(name = "id")
    private HashMap<Integer, Quiz> quizzes;

}

这是我在网上找到的一个例子：https://www.logicbig.com/tutorials/java-ee-tutorial/jpa/map-with-entity-values.html

Answer 1

我认为你试图遵循的方法是不正确的。您不需要有一个map对象来存储关联之间的键和相关实体。

简单的OneTomany映射可以通过以下示例实现：

@Data
@Entity
@Table(name = "users")
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL, orphanRemoval = true, mappedBy = "user")
    private List<Quiz> quizzes = new ArrayList<>();

}

和

@Data
@Entity
@Table(name = "quizzes")
public class Quiz {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @JoinColumn(name = "user_id")
    @ManyToOne(fetch = FetchType.LAZY)
    @ToString.Exclude
    private User user;

}