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")
}

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

还有我的第二套

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

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

我希望通过加入上述两个集合来实现以下输出

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

---

是否可能，如果是，如何进行？

您可以使用

@DBRef

从

学生

中引用

主题

文档

映射框架不必存储嵌入的子对象 在文件中。您还可以单独存储它们并使用 DBRef引用该文档。从中加载对象时 MongoDB，这些参考资料将被急切地解决，您将得到 返回一个看起来与存储的对象相同的映射对象 嵌入到主控文档中

下面是一个使用DBRef引用特定文档的示例 独立于引用它的对象而存在的 （为了简洁起见，两个类都以直线形式显示）：

编辑：-

@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 + "]";
    }
}

@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 + "]";
    }

}

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

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

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

实际上，为了给您提供更多细节，

@DBRef

注释将急切地加载数据（即本例中的主题）

学生示范班：-

@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 + "]";
    }
}

@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 + "]";
    }

}

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

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

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

科目模型班：-

@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 + "]";
    }
}

@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 + "]";
    }

}

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

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

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

通过Id获取学生：-

@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 + "]";
    }
}

@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 + "]";
    }

}

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

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

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

当您通过Id获取学生时，framework/mongodb将自动加载主题数据。您不需要专门执行join来获取主题数据

public Students getStudents(String id) {

        MongoOperations mongoOperations = getMongoConnection();

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

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

        return students;

    }

输出：-

@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 + "]";
    }
}

@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 + "]";
    }

}

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

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

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

主题集合：-

@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 + "]";
    }
}

@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 + "]";
    }

}

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

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

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

学生收藏：-

@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 + "]";
    }
}

@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 + "]";
    }

}

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

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

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

---

您可以这样制作学生收藏：

    students 
{
 "name" : "abc"
 "class" : "1"
 "subjects" : [{ type: Schema.Types.ObjectId}] // You will store Objectids of subject
}

---

现在，当您找到关于学生的查询时，请填充数据。

谢谢您的链接。但是我想执行join来合并数据。有可能吗？当你有了学生，你就有了对象。我希望这就是你所期待的。正如API文档中提到的，它将被急切地加载。我以前用过。对不起，因为我对这个很陌生，你能详细解释一下吗。我只是想知道这个答案是否有用？