Hibernate 一对多映射急切获取不起作用

Question

学院和学生实体之间存在一对多的关系。

大学

@Entity
public class College {

private int collegeId;
private String collegeName;
private List<Student> students;

@Id
@GeneratedValue
public int getCollegeId() {
    return collegeId;
}

public void setCollegeId(int collegeId) {
    this.collegeId = collegeId;
}

public String getCollegeName() {
    return collegeName;
}

public void setCollegeName(String collegeName) {
    this.collegeName = collegeName;
}

@OneToMany(targetEntity=Student.class, mappedBy="college", cascade=CascadeType.ALL, fetch=FetchType.EAGER ) 
public List<Student> getStudents() {
    return students;
}

public void setStudents(List<Student> students) {
    this.students = students;
}

}

学生

@Entity
public class Student {

private int studentId;
private String studentName;
private College college;

@Id
@GeneratedValue
public int getStudentId() {
    return studentId;
}

public void setStudentId(int studentId) {
    this.studentId = studentId;
}

public String getStudentName() {
    return studentName;
}

public void setStudentName(String studentName) {
    this.studentName = studentName;
}

@ManyToOne
@JoinColumn(name="college_id")
public College getCollege() {
    return college;
}

public void setCollege(College college) {
    this.college = college;
}

}

我是休眠新手，所以根据我的理解，如果我将 fetchtype 设置为

FetchType.EAGER

，那么每当我查询单个大学对象时，都会自动获取相关的学生对象。我使用了以下查询，

College college = (College) session.get(College.class, id);

college

对象已正确加载，但是当我说

college.getStudents()

作为回报时，我会得到

null

。我是否遗漏了一些东西，或者这是快速获取的正确方法吗？

Answer 1

我不知道为什么，但在将两个 FetchType 设置为 EAGER 后我可以获取。

学生班

@ManyToOne(fetch=FetchType.EAGER)
private College college;

拼贴.班

@OneToMany(mappedBy="collage",fetch=FetchType.EAGER)
private List<Student> students;

Answer 2

您的代码看起来不错，但是您可以尝试一下并让我们知道它是否有效。

您在 College.java 中的代码行：

@OneToMany(targetEntity=Student.class, mappedBy="college", cascade=CascadeType.ALL, fetch=FetchType.EAGER ) 
public List<Student> getStudents() {
    return students;
}

请尝试将其替换为：

@OneToMany(targetEntity=Student.class, mappedBy="college", cascade=CascadeType.ALL, fetch=FetchType.EAGER ) 
@JoinColumn(name="college_id") // join column is in table for Student
public List<Student> getStudents() {
    return students;
}

希望这对您有帮助。