Question

对于给定的问题，我有以下mongodb集合（questionanswers）

JsonConvert.DefaultSettings = () =>
{
    var settings = new JsonSerializerSettings();
    settings.Converters.Add(new GuidConverterCustom());
    return settings;
};

我需要cnt per sr no，它位于嵌套数组元素中。预计是：

Srno ===＆gt; CNT

1 ===＆gt; 2

2 ===＆gt; 3

我正在尝试以下查询

{ 
"_id" : { "$oid" : "59cda8b730a0131cfafc9101" }, 
"questionId" : { "$oid" : "59cb7b85c854036fec173267" }, 
"userQuestionAnswerDetails" : [ { "srno" : 1, "correct_yn" : false}, { 
 "srno" : 2, "correct_yn" : false} ] 
}

{ 
"_id" : { "$oid" : "59cdb1b930a0131cfafc9102" }, 
"questionId" : { "$oid" : "59cb7b85c854036fec173267" }, 
"userQuestionAnswerDetails" : [ { "srno" : 2, "correct_yn" : false} ] 
}

{ 
"_id" : { "$oid" : "59cdbb1b30a0131cfafc9105" }, 
"questionId" : { "$oid" : "59cb7b85c854036fec173267" }, 
"userQuestionAnswerDetails" : [ { "srno" : 2, "correct_yn" : false} ] 
}

{ 
"_id" : { "$oid" : "59cdd26a30a0131cfafc9107" }, 
"questionId" : { "$oid" : "59cb7b85c854036fec173267" }, 
"userQuestionAnswerDetails" : [ { "srno" : 1, "correct_yn" : false} ] 
}

以上查询给出了类似下面的内容：

db.questionanswers.aggregate([
{
    $match:{
        $and: [
            {questionId: ObjectId("59cb7b85c854036fec173267")}
        ]       
    }
},
{
    $group : {_id : {questionId:"$questionId", userQuestionAnswerDetails: "$userQuestionAnswerDetails.srno"}, num_tutorial : {$sum : 1}}
}
])

相当于 Srno ===＆gt; CNT

1,2 ===＆gt; 1

1 ===＆gt; 1

2 ===＆gt; 2

如何修改$ group以达到预期效果？提前谢谢。

Answer 1

您可以通过添加$unwind阶段来实现。

db.questionanswers.aggregate([
{
    $match:{
        $and: [
            {questionId: ObjectId("59cb7b85c854036fec173267")}
        ]       
    }
}
,{
    $unwind: "$userQuestionAnswerDetails" 
}
,{
    $group : {_id :  {questionId:"$questionId", userQuestionAnswerDetails: "$userQuestionAnswerDetails.srno"}, num_tutorial : {$sum : 1}}
}
])

结果：

{ "_id" : { "questionId" : ObjectId("59cb7b85c854036fec173267"), "userQuestionAnswerDetails" : 2 }, "num_tutorial" : 3 }
{ "_id" : { "questionId" : ObjectId("59cb7b85c854036fec173267"), "userQuestionAnswerDetails" : 1 }, "num_tutorial" : 2 }

嵌套数组元素的mongodb $ group

1 个答案: