为什么在以多对一关系获取数据时执行额外的查询？

Question

I have two entities student and college. A single college has multiple student.

@Entity
public class College {

    @Id
    @GeneratedValue
    private int collegeId;

    private String collegeName;

    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;
    }
}


@Entity
public class Student {

    @Id
    @GeneratedValue
    private int studentId;

    private String studentName;

    @ManyToOne(cascade = CascadeType.ALL)
    private College college;

    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;
    }

    public College getCollege() {
        return college;
    }

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

I want to fetch all the students of a particular college.

As you can see in below code the HQL query I have written is : "select student from "+Student.class.getName()+" student where student.college.collegeId = 1"

On execution of following code I two SQL queries are fired as follows:

Hibernate: select student0_.studentId as studentId1_1_, student0_.college_collegeId as college_collegeId3_1_, student0_.studentName as studentName2_1_ from mevada.Student student0_ where student0_.college_collegeId=1
Hibernate: select college0_.collegeId as collegeId1_0_0_, college0_.collegeName as collegeName2_0_0_ from mevada.College college0_ where college0_.collegeId=?

Ideally first query is sufficient to fetch all required students and it is working well when I fire directly from database.

why second query is executed? How can I stop Hibernate by executing this extra query? Utility class:

public class ManyToOne {

    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("org.hibernate.examples");

        EntityManager em = emf.createEntityManager();

        College college1 = new College();
        college1.setCollegeName("College1");


        College college2 = new College();
        college2.setCollegeName("College2");

        Student student1 = new Student();
        student1.setStudentName("std1");
        student1.setCollege(college1);


        Student student2 = new Student();
        student2.setStudentName("std2");
        student2.setCollege(college2);

        Student student3 = new Student();
        student3.setStudentName("std3");
        student3.setCollege(college1);

        Student student4 = new Student();
        student4.setStudentName("std4");
        student4.setCollege(college1);

        em.getTransaction().begin();

        em.persist(college1);
        em.persist(college2);
        em.persist(student1);
        em.persist(student2);
        em.persist(student3);
        em.persist(student4);

        em.getTransaction().commit();
        em.close();

        em = emf.createEntityManager();
        em.getTransaction().begin();

        String queryString = "select student from "+Student.class.getName()+" student where student.college.collegeId = 1";

        Query query = em.createQuery(queryString);

        List<Student> students = query.getResultList();

        em.getTransaction().commit();
        em.close();
        emf.close();

    }
}

Answer 1

It's because of this

@ManyToOne(cascade = CascadeType.ALL)
private College college;

@ManyToOne is EAGER by default, which means it is populated when Student is fetched from database. You could set the relation to be LAZY , that would delay that second query until you call student.getCollege() . But if you know you'll need college data as well, you should be able to get it all in one query like this

"select student from " + Student.class.getName() + " student inner join student.college col where col.collegeId = 1"