HQL查询问题

Question

I have a problem trying to retrieve items from my database using HQL. I have 2 tables (Student and Classroom) and I want to retrieve all Students together with the Classroom they belong to.

Student table

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

Classroom table

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

Here are the tables as Java classes in case anyone wants to see them.

Student

@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;
    }
}

Classroom

@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;
    }

Now, here is the method with the query I'm trying to use to get the items. I get an exception when I try to call this method.

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;
}

Here is the exception I get.

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

I appreciate all answers and ideas, thanks.

Answer 1

I think you're getting burned by your property names. JavaBeans specifies bean property names like id where the setter/getter are public void setId(long id) and public long getId().

Your current property names are of the form x_id. I would recommend not confusing the property name with the column name. The property name can be id and the column name in the backing table can be class_id for the Class (poorly named as it conflicts with java.lang.Class).

ADDITIONAL EDIT

@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 2

In fact there is no field named classroomId on the Student Entity. How should hibernate know about? Is it really a column in your database? If yes, how did it get there?

And is it correct that the type of Student.studentClass is just Class? Is it a custom type or does it refer to java.lang.Class? It should instead point to your Classroom type, shouldn't it? If that is the case, @JoinColumn(name = "class_id") should be @JoinColumn(name = "classroom_id") instead. But then the join column annotation is not needed at all because hibernate knows how to join.

Then you could optimize your query to something like:

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

Answer 3

在HQL中，您使用实体而不是表-因此，由于学生包含教室，因此hql查询应为：

Select student from Student student Join Fetch student.classroom