MongoDB:如何从$ Elemmatch中返回/仅访问一个文档

Question

说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。我想返回 在学生字段中具有给定学生ID的课程。

如果我使用$elemMatch，我将在课程集合以及我想要的课程中获得很多文档。有没有办法解决这个问题，或者我绝对必须进行一些查询过滤才能返回一个文档？

编辑：示例课程集合：

{
 _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 }
           ]
}

如果我使用$elemMatch查找课程，则预期输出Jeff正在参加：

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

vers

{
 _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

我只想返回学生字段中具有给定学生ID的课程

您可以在查询中使用$elemMatch 根据嵌入式文档中字段的值选择文档。然后使用简单的投影，您可以返回感兴趣的字段：

> 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，如果您需要整个文档，请勿指定任何投影：

> 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，这将返回所有课程，其中有一个具有匹配名称的学生。在我们的示例数据中，" jeff" 仅与一门课程相关联。但这将返回两个文档，" John" 。

如果您只想要其中一个，则可以将聚合框架与$limit阶段一起使用（在大多数情况下，您应该在$limit之前使用$sort阶段）：

> 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
        }
    ]
}