HQL查询

Question

问题我尝试使用HQL从我的数据库检索项目时遇到问题。我有两张桌子（学生和教室），我想要将所有学生和他们所属的课室一起找回。

学生表

user_id | name | nick | password | surname | classroom_id 
---------|-----------|-----------|-----------|-----------|-------------- 
    2  | AAA  | studentA | test123 | AAA  |  1 
    3  | BBB  | studentB | test321 | BBB  |  1 

课堂表

classroom_id | name  | year 
--------------|------------|---------- 
     1  | TestClass | 2016/2017 

下面是表作为的情况下，任何人都希望看到他们的Java类。

学生

@Entity 
public class Student extends User{ 

    private Class studentClass; 

    public Student() { 
    } 

    public Student(String name, String surname, String nick, String password, Class studentClass) { 
     super(name, surname, nick, password); 
     this.studentClass = studentClass; 
    } 

    @ManyToOne 
    @JoinColumn(name = "classroom_id") 
    public Class getStudentClass() { 
     return studentClass; 
    } 

    public void setStudentClass(Class studentClass) { 
     this.studentClass = studentClass; 
    } 
} 

课堂

@Entity 
public class Classroom { 

    private SimpleStringProperty name = new SimpleStringProperty(); 
    private SimpleStringProperty year = new SimpleStringProperty(); 

    private int classroom_id; 

    public Classroom() { 
    } 

    public Classroom(String name, String year) { 
     this.name.set(name); 
     this.year.set(year); 
    } 


    @Column(nullable = false) 
    public String getName() { 
     return name.get(); 
    } 

    public SimpleStringProperty nameProperty() { 
     return name; 
    } 

    public void setName(String name) { 
     this.name.set(name); 
    } 

    @Column(nullable = false) 
    public String getYear() { 
     return year.get(); 
    } 

    public SimpleStringProperty yearProperty() { 
     return year; 
    } 

    public void setYear(String year) { 
     this.year.set(year); 
    } 


    @Id 
    @GenericGenerator(name="id" , strategy="increment") 
    @GeneratedValue(generator="id") 
    public int getClassroom_id() { 
     return classroom_id; 
    } 

    public void setClassroom_id(int classroom_id) { 
     this.classroom_id = classroom_id; 
    } 

现在，这里是与查询我试图用它来获得的项目的方法。我尝试调用此方法时遇到异常。

public List<Object> getAllStudentsWithClass(){ 
    Session session = HibernateUtil.getSessionFactory().getCurrentSession(); 
    Transaction transaction = session.beginTransaction(); 
    String select = "SELECT student, classroom FROM Student student, Classroom classroom WHERE student.classroom_id = classroomm.classroom_id"; 
    Query query = session.createQuery(select); 
    List<Object> list = query.list(); 
    return list; 
} 

这是我得到的例外。

Exception in thread "JavaFX Application Thread" org.hibernate.QueryException: could not resolve property: classroom_id of: model.Student [SELECT student, classroom FROM model.Student student, model.Classroom classroom WHERE student.classroom_id = classroomm.classroom_id] 

Caused by: org.hibernate.QueryException: could not resolve property: classroom_id of: model.Student 

我感谢所有的答案和想法，谢谢。

Answer 1

在HQL，你的实体，而不是表的工作 - 因此，由于学生包含了教室HQL查询应该是：

Select student from Student student Join Fetch student.classroom 

Answer 2

我想你会被你的财产名称烧毁。 JavaBeans指定bean的属性名称，如id，setter/getter是public void setId（long id）和public long getId（）。

您当前的属性名称格式为x_id。我建议不要将属性名称与列名混淆。属性名称可以是id，并且支持表中的列名可以是Class的class_id（因为它与java.lang.Class冲突，所以名称很差）。

附加编辑

@Entity 
public class Classroom { 

    private SimpleStringProperty name = new SimpleStringProperty(); 
    private SimpleStringProperty year = new SimpleStringProperty(); 

    // Follow JavaBeans naming conventions 
    private int classroomId; 

    public Classroom() { 
    } 

    public Classroom(String name, String year) { 
     this.name.set(name); 
     this.year.set(year); 
    } 


    @Column(nullable = false) 
    public String getName() { 
     return name.get(); 
    } 

    public SimpleStringProperty nameProperty() { 
     return name; 
    } 

    public void setName(String name) { 
     this.name.set(name); 
    } 

    @Column(nullable = false) 
    public String getYear() { 
     return year.get(); 
    } 

    public SimpleStringProperty yearProperty() { 
     return year; 
    } 

    public void setYear(String year) { 
     this.year.set(year); 
    } 


    @Id 
    @GenericGenerator(name="id" , strategy="increment") 
    @GeneratedValue(generator="id") 
    public int getClassroomId() { 
     return classroomId; 
    } 

    public void setClassroomId(int classroomId) { 
     this.classroomId = classroomId; 
    } 
} 

Answer 3

其实没有现场对学生实体命名classroomId。 hibernate应该怎么知道？它真的是数据库中的一列吗？如果是的话，它是如何到达那里的？

这是正确的，Student.studentClass的类型只是类？它是一个自定义类型还是引用java.lang.Class？它应该指向您的“课堂”类型，不是吗？如果是这样的话，@JoinColumn（name =“class_id”）应该是@JoinColumn（name =“classroom_id”）。但是，因为hibernate知道如何连接，所以根本不需要连接列注释。

然后，你可以优化您的查询是这样的：

SELECT ... FROM Student s JOIN s.studentClass c WHERE ...