多对一关系不填充新实体答案

【问题标题】：ManyToOne relationship does not populate new entity多对一关系不填充新实体
【发布时间】：2021-12-14 11:47:42
【问题描述】：

我的目标是创建一个新实体来处理 @ManyToMany 关系的问题。我想审核新实体，但休眠没有用数据填充 CourseRegistration-new 实体。

@Entity
@Table(name = "course_registration")
public class CourseRegistration {

    @Id
    @Column(name = "id")
    private Long id;

    @ManyToOne
    @JoinColumn(name = "student_id")
    private Student student;

    @ManyToOne
    @JoinColumn(name = "course_id")
    private Course course;

    public CourseRegistration() {}
}


@Entity
@Table(name = "student")
public class Student {

    @Id
    @Column(name = "id")
    private Long id;

    @OneToMany(mappedBy = "student")
    private List<CourseRegistration> registrations = new ArrayList<>();

    public Student() {}
}

@Entity
@Table(name = "course")
public class Course {

    @Id
    @Column(name = "id")
    private Long id;

    @OneToMany(mappedBy = "course")
    private List<CourseRegistration> registrations = new ArrayList<>();

    public Course() {}
}

【问题讨论】：

请指定哪些数据未填充，请提供您将值传递给休眠的服务方法。

标签： java spring hibernate jpa orm

【解决方案1】：

您必须在映射中提及获取类型，如果未添加到您的实体中，还必须添加 getter 和 setter，如下所示。

@Entity
@Table(name = "course_registration")
@Data
@NoArgsConstructor
@AllArgsConstructor
public class CourseRegistration {

    @Id
    @Column(name = "id")
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    @JsonIgnore
    @JoinColumn(name = "student_id")
    private Student student;

    @ManyToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    @JsonIgnore
    @JoinColumn(name = "course_id")
    private Course course;

}


@Entity
@Table(name = "student")
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Student {

    @Id
    @Column(name = "id")
    private Long id;

    @OneToMany(mappedBy = "student")
    private List<CourseRegistration> registrations = new ArrayList<>();
    
}

@Entity
@Table(name = "course")
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Course {

    @Id
    @Column(name = "id")
    private Long id;

    @OneToMany(mappedBy = "course")
    private List<CourseRegistration> registrations = new ArrayList<>();
    
}

添加以下依赖项以使用 lambok 启用 getter 和 setter。

<dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>

【讨论】：

【解决方案2】：

我敢打赌，您的 @OneToMany 注释上缺少级联选项。请尝试以下操作：

@Entity
@Table(name = "course")
public class Course {

    @Id
    @Column(name = "id")
    private Long id;

    @OneToMany(mappedBy = "course", cascade = CascadeType.ALL)
    private List<CourseRegistration> registrations = new ArrayList<>();

    public Course() {}
}

ALL 表示将以下操作传播到关联实体（在本例中为 CourseRegistration）：PERSIST、MERGE、REMOVE、REFRESH 和 DETACH。

【讨论】：