Spring Hibernate Mysql：如何获取多对一，多对多，一对多的对象

Question

For the relationship of many-to-one, one-to-many or even many-to-many, how to get an object without the objects included on the other side?

Say a group of address and a group of people, the relationship would be many-to-many, what if i just wanne get a "people" without the concerned address?

Classroom.java

@Entity
public class Classroom {

public Classroom(){
}

public Classroom(long id, String name) {
    this.id = id;
    this.name = name;
}

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "id")
private long id;

@Column(name = "name")
private String name;

@ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
@JoinTable(name = "classroom_people", 
        joinColumns = {@JoinColumn(name = "classroom_id", referencedColumnName = "id") }, 
        inverseJoinColumns = {@JoinColumn(name = "people_id", referencedColumnName = "id") })
private List<People> peoples = new ArrayList<>();

... 
// getter() and setter()

}

People.java

@Entity
public class People{

public People(){
}

public People(long id, String name) {
    this.id = id;
    this.name = name;
}

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name="id")
private long id;

@Column(name = "name")
private String name;

@ManyToMany(mappedBy = "peoples", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
private List<Classroom> classrooms = new ArrayList<>();

...
// getter() and setter()

}

By using hibernate, the table of Classroom, People and the join table classroom_people have been created automatically. Until now, everything is fine.

Then by using spring repository, I tried to fetch a people by passing the name, and return a JSON object.

What I hope to get is just a JSON including people.id and people.name but what I get is a JSON including one people and all the classroom in the Set, which includes all the concerning peoples .

SO I guess the problem is the List and List and what I should do is to create three tables: classroom, people and classroom_people, then store the relationship manually to join table. Then I can fetch a simple object without problem.

AM I right?

I use Spring Repository and findAllByName(), so the SQL is generated by spring. Just wonder whether this would be the source of problem.

Answer 1

It's easily done with HQL if you keep the relationships bidirectional. Something like:

String hql = "select addr.people from " + Address.class.getName() + " addr where addr.name(or some other attribute) = :addressName"
//set parameter "addressName"

EDIT: Now you took a different example, but the above query applies the same, for example getting people from a classroom.

About your last note, getting a People by name will fetch only People instance, because the @ManyToMany List referencing classrooms is LAZY which means is not populated until you call people.getClassRooms() (if you have the session open it will load the empty list with another query) You don't need to create the junction table yourselves, not for this purpose.

How do you know that List<ClassRoom> is populated when you fetch a People ? Because from your mappings it looks OK, unless you're calling people.getClassRooms()