Node.js 如何返回从MongoDb中的一个数组筛选出的多个数组

我在MongoDb数据库中有一些数据，正在使用NodeJS MongoDb本机驱动程序访问它

user: {
    _id: ****Example Id****
    name: 'Foo bar',
    examScores: [
        { subject: 'Geography', score: 80, teacher: 'Mr Foo', date: 'somedate'},
        { subject: 'History', score: 57, teacher: 'Mrs Bar', date: 'somedate'},
        { subject: 'Maths', score: 43, teacher: 'Mrs Fizz', date: 'somedate'},
        { subject: 'Geography', score: 43, teacher: 'Mr Buzz', date: 'somedate'},
        { subject: 'Geography', score: 78, teacher: 'Mr Foo', date: 'somedate'},
        { subject: 'History', score: 41, teacher: 'Mr Buzz', date: 'somedate'}
    ]
}

我想检索每个科目的前十名分数/考试。为了简洁起见，我在这个例子中没有包括超过十次的考试尝试

比如：

user: {
    _id: ****Example Id****
    name: 'Foo bar',
    grades: [
        { subject: 'Geography', exams: [ 
            { score: 80, teacher: 'Mr Foo', date: 'somedate' },
            { score: 78, teacher: 'Mr Foo', date: 'somedate' },
            { score: 43, teacher: 'Mr Buzz', date: 'somedate' }
            ]
        },
        { subject: 'History', exams: [
            { score: 57, teacher: 'Mrs Bar', date: 'somedate' },
            { score: 41, teacher: 'Mr Buzz', date: 'somedate'}
            ]
        },
        { subject: 'Maths', exams: [
            { score: 43, teacher: 'Mrs Fizz', date: 'somedate'}
            ]
        }
    ]
}       

---

请尝试此查询，希望能有所帮助。我使用了，然后聚合数据

db.collection.aggregate([ {$unwind:"$examScores"}, 

{$group:{"_id":"$examScores.subject",name: { $first: "$name" }, exams: { 

$addToSet:{ teacher: '$examScores.teacher' 

,score:'$examScores.score',date:'$examScores.date' } }}},

{$group:{"_id":null,name: { $first: "$name" }, "grades":{$push: 

{"subject":"$_id", "exams":"$exams"}}}},

{$project:{"_id":0,"grades":1,"name":1}} ])

我得到的预期输出在本地机器上

{
  "name": "Foo bar",
  "grades": [
    {
      "subject": "History",
      "exams": [
        {
          "teacher": "Mr Buzz",
          "score": 41,
          "date": "somedate"
        },
        {
          "teacher": "Mrs Bar",
          "score": 57,
          "date": "somedate"
        }
      ]
    },
    {
      "subject": "Maths",
      "exams": [
        {
          "teacher": "Mrs Fizz",
          "score": 43,
          "date": "somedate"
        }
      ]
    },
    {
      "subject": "Geography",
      "exams": [
        {
          "teacher": "Mr Foo",
          "score": 78,
          "date": "somedate"
        },
        {
          "teacher": "Mr Buzz",
          "score": 43,
          "date": "somedate"
        },
        {
          "teacher": "Mr Foo",
          "score": 80,
          "date": "somedate"
        }
      ]
    }
  ]
}

---

请尝试以下查询：

展开ExamCores阵列

根据主题和分数按降序对集合进行排序 你需要每个科目的成绩按分数降序排列

基于主题的分组结果

因为你只需要列出每个科目的前10名最高分数 您可以使用操作限制前10个最高分 每一个科目

db.collection.aggregate([ 

               { $unwind : "$examScores" },
               { $sort : 
                       { "examScores.subject" : 1, 
                         "examScores.score" : -1
                       }
               }, 
               { $group : { 
                          "_id" : "$examScores.subject",
                          "name" : { $first: "$name" }, 
                          "examsAll" : { 
                                 $addToSet : { 
                                        teacher: '$examScores.teacher',
                                        score : '$examScores.score',
                                        date : '$examScores.date' 
                                             } 
                                       }
                            }
               },
               { $project : { 
                             "_id" : 1, 
                             "name" : 1, 
                             "exams" : { $slice: [ "$examsAll", 10 ] } 
                            }
               },
               { $group : { "_id" : null, 
                            "name" :  { $first: "$name" }, 
                            "grades" : { $push: 
                                             { "subject" : "$_id", 
                                               "exams" : "$exams" 
                                             }
                                       }
                           }
               },
               { $project : { "_id" : 0,
                              "grades" : 1,
                              "name" : 1
                            }
               } 
     ]);