为什么我的mongodb查询不返回任何内容？

Question

/*我是MongoDB的新手，并试图找出如何获得CSIS2300的平均correct_answers。

There is a part of the document. Some users take more than one course */ 


     { 
    "_id" : 1.0, 
    "user_id" : "jjackson0", 
    "first_name" : "Jack", 
    "last_name" : "Jackson", 
    "email" : "jjackson0@apache.org", 
    "status" : "active", 
    "join_date" : "2014-12-10", 
    "last_login_date" : "2016-09-30 23:51:41 -0400", 
    "strengths" : [ 
     "mongo queries", 
     "mongo map-reduce queries" 
    ], 
    "courses" : [ 
     { 
      "code" : "CSIS2300", 
      "total_questions" : 165.0, 
      "correct_answers" : 153.0, 
      "incorect_answers" : 12.0 
     } 
    ] 
} 

/* 2 */ 
{ 
    "_id" : 2.0, 
    "user_id" : "ecoleman1", 
    "first_name" : "Eugene", 
    "last_name" : "Coleman", 
    "email" : "ecoleman1@techcrunch.com", 
    "age" : 49.0, 
    "status" : "banned", 
    "join_date" : "2015-07-01", 
    "last_login_date" : "2016-09-30 23:54:08 -0400", 
    "address" : { 
     "city" : "Chencun", 
     "province" : "PEI" 
    }, 
    "strengths" : [ 
     "visualization", 
     "sql", 
     "query optimisation", 
     "dimensional modelling", 
     "analytics research" 
    ], 
    "courses" : [ 
     { 
      "code" : "CSIS2300", 
      "total_questions" : 188.0, 
      "correct_answers" : 106.0, 
      "incorect_answers" : 82.0 
     }, 
     { 
      "code" : "CSIS3300", 
      "total_questions" : 12.0, 
      "correct_answers" : 9.0, 
      "incorect_answers" : 3.0 
     }, 
     { 
      "code" : "CSIS3380", 
      "total_questions" : 172.0, 
      "correct_answers" : 142.0, 
      "incorect_answers" : 30.0 
     }, 
     { 
      "code" : "CSIS3360", 
      "total_questions" : 140.0, 
      "correct_answers" : 21.0, 
      "incorect_answers" : 119.0 
     }, 
     { 
      "code" : "CSIS4260", 
      "total_questions" : 163.0, 
      "correct_answers" : 75.0, 
      "incorect_answers" : 88.0 
     } 
    ] 
} 

/* 3 */ 
{ 
    "_id" : 3.0, 
    "user_id" : "mbowman2", 
    "first_name" : "Mark", 
    "last_name" : "Bowman", 
    "email" : "mbowman2@ebay.co.uk", 
    "age" : 36.0, 
    "status" : "disabled", 
    "join_date" : "2015-02-01", 
    "last_login_date" : "2016-09-30 23:58:07 -0400", 
    "address" : { 
     "city" : "Ban Kruat", 
     "province" : "NL" 
    }, 
    "strengths" : [ 
     "mongo map-reduce queries", 
     "sql", 
     "dimensional modelling", 
     "visualization" 
    ], 
    "courses" : [ 
     { 
      "code" : "CSIS2300", 
      "total_questions" : 185.0, 
      "correct_answers" : 171.0, 
      "incorect_answers" : 14.0 
     }, 
     { 
      "code" : "CSIS3300", 
      "total_questions" : 57.0, 
      "correct_answers" : 54.0, 
      "incorect_answers" : 3.0 
     } 
    ] 
} 

/* 4 */ 
{ 
    "_id" : 4.0, 
    "user_id" : "acollins3", 
    "first_name" : "Andrew", 
    "last_name" : "Collins", 
    "email" : "acollins3@unesco.org", 
    "status" : "inactive", 
    "join_date" : "2016-03-06", 
    "last_login_date" : "2016-09-30 23:59:05 -0400", 
    "strengths" : [ 
     "analytics research", 
     "mongo queries", 
     "normalization" 
    ], 
    "courses" : [ 
     { 
      "code" : "CSIS2300", 
      "total_questions" : 101.0, 
      "correct_answers" : 37.0, 
      "incorect_answers" : 64.0 
     } 
    ] 
} 

有我的查询不返回任何东西。

db.assign1.aggregate([ 
      {$project: 
       {_id: 0, 
       course_code: "$courses.code", 
       correctAns: {$max: "$courses.correct_answers"}}}, 
      {$match: {"courses.code": "CSIS2300"}}, 
      {$group: {_id: {"code": "$courses.code"}, Average_CorrectAns: {$avg: "$courses.correct_answers"}}} 

      ]); 

Answer 1

因为你$项目阶段改变$ courses.code到course_code，所以这个阶段后也没有什么领域使用该名称（同样适用于$ courses.correct_answers）。另外，通常最好先与$匹配，因为它可以提高查询的性能。此外，您可以逐个将查询阶段添加到查询中，并每次运行查看每个阶段后获得的输出。

要查找平均需要$放松身心的课程阵列，最终查询如下：

db.assign1.aggregate([{$unwind: "$courses"}, 
         {$match: {"courses.code": "CSIS2300"}}, 
         {$group: {_id: {"code": "$courses.code"}, correctAns: {$avg: "$courses.correct_answers"}}}, 
         {$project: {course_Code: "$_id.code", correctAns: 1, _id: 0}} 
        ]);