为什么我的单向@ManyToOne关系中的表未加入Spring Data findAll（）查询？

Question

我有一个具有[[大学和学生的简单数据库（请参见下面的实体）

University.java

import lombok.Getter;
import lombok.Setter;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Getter
@Setter
@Table(name = "university")
public class University {

    @Id
    @GeneratedValue
    @Column(name = "university_id")
    private long id;

    @Column(name = "university_name")
    private String name;

    public University() {
    }

    public University(String name) {
        this.name = name;
    }
}

Student.java

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Entity
@Getter
@Setter
@Table(name = "student")
public class Student {
    @Id
    @GeneratedValue
    @Column(name = "student_id")
    private long id;

    @Column(name = "student_name")
    private String studentName;

    @JoinColumn(name="university_name")
    private String studentUniversityName;

    @ManyToOne
    private University university;

    public Student() {
    }

    public Student(String name, String studentUniversityName) {
        this.name = name;
        this.studentUniversityName = studentUniversityName;
    }
}

我的实体应在university_name列上加入。我编写了以下单元测试来验证此行为。
    @Test
    void test1() {
        final String UNIVERSITY_NAME = "U of A";

        universityRepository.save(new University(UNIVERSITY_NAME));

        studentRepository.save(new Student("Joe", UNIVERSITY_NAME));

        Student joe = studentRepository.findAll().get(0);
        assertNotNull(joe.getUniversity()); // <------ FAILING
    }

assertNotNull(joe.getUniversity())语句失败，这似乎表明表未联接。对我来说，奇怪的是，下一个测试通过了（当我明确地将大学设置为Joe时。（数据库的内容在两种方法中都是相同的，但是此测试的findAll()似乎正确触发了连接）
  @Test
    void test2() {
        final String UNIVERSITY_NAME = "U of A";

        University uni = new University(UNIVERSITY_NAME);
        universityRepository.save(uni);

        Student joe1 = new Student("Joe", UNIVERSITY_NAME);
        joe1.setUniversity(uni);
        studentRepository.save(joe1);

        Student student = studentRepository.findAll().get(0);
        assertNotNull(student.getUniversity());
    }

我不想在救他之前将大学设置为Joe（就像在第二个测试中一样），因为有时我想在代码中的不同时间保存学生和大学。我希望能够用空大学来保存Joe并在对findAll()的调用中填充字段
有人可以帮我修改我的实体，以便通过第一项测试吗？

Answer 1

而不是在Student类中创建另一个字段StudentUniversityName，并在大学类中添加@onetomany。

public class Student {
@Id
@GeneratedValue
@Column(name = "student_id")
private long id;

@Column(name = "student_name")
private String studentName;

@ManyToOne
@JoinColumn(name="university_name")    
private University university;

public Student() {
}

public Student(String name, String studentUniversityName) {
    this.name = name;
    this.studentUniversityName = studentUniversityName;
}
}

public class University {

@Id
@GeneratedValue
@Column(name = "university_id")
private long id;

@Column(name = "university_name")
private String name;

@OneToMany(mappedBy="university")
private List<Student> students= new ArrayList<Student>();

public University() {
}

public University(String name) {
    this.name = name;
}
}