JPA實體是否可以有多個OneToMany關聯？

Question

將兩個OneToMany關聯添加到我的實體類似乎不起作用。如果我刪除其中一個，它工作正常。

@Entity 
@Table(name = "school") 
public class School { 

    private List<Teacher> teachers; 
    private List<Student> students; 

    @OneToMany(cascade=CascadeType.ALL, mappedBy = "school", fetch = FetchType.EAGER) 
    public List<Teacher> getTeachers() 
     return this.teachers; 
    } 
    public void setTeachers(List<Teacher> teachers) { 
     this.teachers = teachers; 
    } 

    @OneToMany(cascade=CascadeType.ALL, mappedBy = "school", fetch = FetchType.EAGER) 
    public List<Student> getStudents() 
     return this.students; 
    } 
    public void setStudents(List<Student> teachers) { 
     this.students = students; 
    } 
} 

然後在Teacher和Student我有正確的ManyToOne註釋

@Entity 
@Table(name = "teacher") 
public class Teacher { 
    private School school; 

    @ManyToOne 
    @JoinColumn(name = "school_id") 
    public School getSchool() { 
     return this.school; 
    } 
    public void setSchool(School school) { 
     this.school = school; 
    } 
} 

@Entity 
@Table(name = "student") 
public class Student { 
    private School school; 

    @ManyToOne 
    @JoinColumn(name = "school_id") 
    public School getSchool() { 
     return this.school; 
    } 
    public void setSchool(School school) { 
     this.school = school; 
    } 
} 

我也有id領域與正確的註釋（@Id，@GeneratedValue）

所以對我來說好像我在同一班級中不能有多於一個@OneToMany。它是否正確？

Answer 1

只要only one is EAGER可以有多個一對多關聯。

雖然您可以使用Set instead of List繞過此異常，但這不是一件好事，因爲您最終會得到一個笛卡爾積。

Best thing to do is to not use EAGER at all。您應該對所有關聯使用LAZY，並且熱切希望獲取與query-time FETCH directive關聯的many-to-one和one-to-one關聯以及最多一個one-to-many關聯。

如果您想要獲取多個one-to-many關聯，則應該使用方法進行第2或第3關聯，因爲3個簡單的select語句優於笛卡爾乘積。