如何在MongoDB中

Question

MongoDB的小白在這裏打最小結果...

所以，我想打印出一個集合，看起來像這裏面的最小值得分...

 > db.students.find({'_id': 1}).pretty() 
     { 
       "_id" : 1, 
       "name" : "Aurelia Menendez", 
       "scores" : [ 
         { 
           "type" : "exam", 
           "score" : 60.06045071030959 
         }, 
         { 
           "type" : "quiz", 
           "score" : 52.79790691903873 
         }, 
         { 
           "type" : "homework", 
           "score" : 71.76133439165544 
         }, 
         { 
           "type" : "homework", 
           "score" : 34.85718117893772 
         } 
       ] 
     } 

我使用的咒術這樣...

db.students.aggregate(
    // Initial document match (uses index, if a suitable one is available) 
    { $match: { 
     _id : 1 
    }}, 

    // Expand the scores array into a stream of documents 
    { $unwind: '$scores' }, 

    // Filter to 'homework' scores 
    { $match: { 
     'scores.type': 'homework' 
    }}, 

    // grab the minimum value score 
    { $match: { 
     'scores.min.score': 1 
    }} 
) 

我得到的輸出是這樣的......

{ "result" : [ ], "ok" : 1 } 

我在做什麼錯？

Answer 1

你有正確的想法，但在聚合的最後一步，你想要做的是按學生分組所有分數，並找到$最小值。

更改最後一個流水線操作：

{ $group: { 
     _id: "$_id", 
     minScore: {$min: "$scores.score"} 
    }} 

Answer 2

> db.students.aggregate(  
{ $unwind: "$scores" },` 
{ $match:{"scores.type":"homework"} }, 
{ $group: { 
    _id : "$_id", 
    maxScore : { $max : "$scores.score"}, 
    minScore: { $min:"$scores.score"} 
    } 
}); 

how to aggregate on each item in collection in mongoDB