MongoDB：如何返回/访问$ elemMatch中的一个文档

Question

假设我有一个学生对象数组字段，例如[{id:foo, name: bar, imageurl:baz}]在课程集合中。并且我想仅返回在学生字段中具有给定学生ID的课程。

如果我使用$elemMatch，我会在课程集合以及我想要的课程中获得一堆文档。有没有办法解决这个问题，或者我绝对必须做一些后期查询过滤来只返回一个文档？

编辑： 例如课程集合：

{ 
_id: 1, 
course: "OOP 101", 
students: [ 
       { name: "ajax", school: 100, age: 7 }, 
       { name: "achilles", school: 100, age: 8 }, 
      ] 
} 
{ 
_id: 2, 
course: "Programming 101", 
students: [ 
       { name: "john", school: 102, age: 10 }, 
       { name: "ajax", school: 100, age: 7 }, 
       { name: "achilles", school: 100, age: 8 }, 
      ] 
} 
{ 
_id: 3, 
course: "Database 101", 
students: [ 
       { name: "john", school: 102, age: 10 }, 
       { name: "jess", school: 102, age: 11 }, 
       { name: "jeff", school: 108, age: 15 } 
      ] 
} 

预计如果我使用$elemMatch找到课程杰夫正在输出：

{ 
_id: 3, 
course: "Database 101", 
students: [ 
       { name: "john", school: 102, age: 10 }, 
       { name: "jess", school: 102, age: 11 }, 
       { name: "jeff", school: 108, age: 15 } 
      ] 
} 

与

{ 
_id: 1, 
course: "OOP 101" 
{ 
_id: 2, 
course: "Programming 101" 
} 
{ 
_id: 3, 
course: "Database 101", 
students: [ 
       { name: "john", school: 102, age: 10 }, 
       { name: "jess", school: 102, age: 11 }, 
       { name: "jeff", school: 108, age: 15 } 
      ] 
} 

Answer 1

我想仅返回t他在学生领域中具有给定学生ID的课程

您可以使用$elemMatchin a query根据嵌入文档中字段的值选择文档。然后，使用一个简单的推算，你可以回到感兴趣仅领域：

> db.test.courses.find(
    {students: {$elemMatch : { name: "john"}}}, // find all courses having 
                // "john" as a student 
    {course:1})         // keep only the course name 
                // and the _id 
{ "_id" : 2, "course" : "Programming 101" } 
{ "_id" : 3, "course" : "Database 101" } 

---

当然，如果您需要整个文件，不指定任何预测：

> db.test.courses.find(  {students: {$elemMatch : { name: "jeff"}}}).pretty() 
{ 
    "_id" : 3, 
    "course" : "Database 101", 
    "students" : [ 
     { 
      "name" : "john", 
      "school" : 102, 
      "age" : 10 
     }, 
     { 
      "name" : "jess", 
      "school" : 102, 
      "age" : 11 
     }, 
     { 
      "name" : "jeff", 
      "school" : 108, 
      "age" : 15 
     } 
    ] 
} 

请注意，这将返回全部有一个学生与匹配的名称的课程。在我们的样本数据中，“jeff”仅与一个课程相关联。但这会返回两个文件“john”。至少

> db.test.courses.aggregate([ 
     {$match:{students: {$elemMatch : { name: "john"}}}}, 
     {$limit:1} 
    ]).pretty() 
{ 
    "_id" : 2, 
    "course" : "Programming 101", 
    "students" : [ 
     { 
      "name" : "john", 
      "school" : 102, 
      "age" : 10 
     }, 
     { 
      "name" : "ajax", 
      "school" : 100, 
      "age" : 7 
     }, 
     { 
      "name" : "achilles", 
      "school" : 100, 
      "age" : 8 
     } 
    ] 
}