java – 如何在mongo中连接DBRef对象

嗨,我有我的第一个收藏

    students 
{
 "name" : "abc"
 "class" : "1"
 "subjects" : DBRef("subjects","class1")
}

和我的第二个系列

 subjects
{
 "_id" : "class1"
 "sub1" : "english"
 "sub2" : "physics"
}

我希望在加入上述两个集合之后实现我的输出

   {
 "name" : "abc"
 "class" : "1"
 "subjects" : {sub1 : "english",sub2 : "physics"}
}

有可能,如果是的话怎么样？

解决方法:

您可以使用@DBRef从Student引用Subjects文档.

The mapping framework doesn’t have to store child objects embedded
within the document. You can also store them separately and use a
DBRef to refer to that document. When the object is loaded from
MongoDB, those references will be eagerly resolved and you will get
back a mapped object that looks the same as if it had been stored
embedded within your master document.

Here’s an example of using a DBRef to refer to a specific document
that exists independently of the object in which it is referenced
(both classes are shown in-line for brevity’s sake):

Refer this link

Another SO link

编辑：-

实际上,为了给你更多的细节,@ DBRef注释会急切地加载数据(在这种情况下是主题).

学生模特课： –

@Document(collection = "students")
public class Students implements Serializable, BaseDocument {

    private static final long serialVersionUID = -3534650012619938612L;

    @Id
    private String id;

    @Field("class")
    private String className;

    @DBRef
    @Field("subjects")
    private Subject subject;

    public String getId() {
        return id;
    }

    public String getClassName() {
        return className;
    }

    public void setId(String id) {
        this.id = id;
    }

    public void setClassName(String className) {
        this.className = className;
    }

    public Subject getSubject() {
        return subject;
    }

    public void setSubject(Subject subject) {
        this.subject = subject;
    }

    @Override
    public String toString() {
        return "Students [id=" + id + ", className=" + className + ", subject=" + subject + "]";
    }
}

科目类型： –

@Document(collection = "subjects")
public class Subject implements Serializable, BaseDocument {

    private static final long serialVersionUID = -3534650012619938612L;

    @Id
    private String id;

    private String sub1;
    private String sub2;

    public String getId() {
        return id;
    }

    public String getSub1() {
        return sub1;
    }

    public String getSub2() {
        return sub2;
    }

    public void setId(String id) {
        this.id = id;
    }

    public void setSub1(String sub1) {
        this.sub1 = sub1;
    }

    public void setSub2(String sub2) {
        this.sub2 = sub2;
    }

    @Override
    public String toString() {
        return "Subject [id=" + id + ", sub1=" + sub1 + ", sub2=" + sub2 + "]";
    }

}

按身份获取学生： –

当您通过Id获得学生时,框架/ mongodb将自动加载主题数据.您无需专门进行连接即可获取主题数据.

public Students getStudents(String id) {

        MongoOperations mongoOperations = getMongoConnection();

        Students students = mongoOperations.findById(id, Students.class);

        System.out.println(students.toString());

        return students;

    }

输出： –

Students [id=584ea66e9e53b7802651de36, className=1, subject=Subject [id=class1, sub1=english, sub2=physics]]

科目集： –

{
    "_id" : "class1",
    "sub1" : "english",
    "sub2" : "physics"
}

学生收藏： –

{
    "_id" : ObjectId("584ea66e9e53b7802651de36"),
    "name" : "abc",
    "class" : "1",
    "subjects" : {
        "$ref" : "subjects",
        "$id" : "class1",
        "$db" : "localhost"
    }
}