从MongoDB中的数组中获取随机元素

Question

这是我的数据库结构

{
     "_id" : ObjectId("576155226d1d298c2cc3edca"),
     "questionLibrary" : {
             "technologyName" : "CSS",
             "questions" : [
                     {
                             "correctanswer" : {
                                     "A1" : "CSS1"
                             },
                             "answeroption" : {
                                     "A4" : "CSS1",
                                     "A3" : "CSS1",
                                     "A2" : "CSS1",
                                     "A1" : "CSS1"
                             },
                             "level" : "Amature",
                             "tags" : "CSS",
                             "question" : "CSS1"
                     },
                     {
                             "question" : "CSS2",
                             "tags" : "CSS",
                             "answeroption" : {
                                     "A1" : "CSS2",
                                     "A2" : "CSS2",
                                     "A3" : "CSS2",
                                     "A4" : "CSS2"
                             },
                             "level" : "Amature",
                             "correctanswer" : {
                                     "A1" : "CSS2"
                             }
                     },
                     {
                             "correctanswer" : {
                                     "A1" : "CSS3"
                             },
                             "answeroption" : {
                                     "A4" : "CSS3",
                                     "A3" : "CSS3",
                                     "A2" : "CSS3",
                                     "A1" : "CSS3"
                             },
                             "level" : "Amature",
                             "tags" : "CSS",
                             "question" : "CSS3"
                     },
                     {
                             "correctanswer" : {
                                     "A1" : "CSS4"
                             },
                             "answeroption" : {
                                     "A4" : "CSS4",
                                     "A3" : "CSS4",
                                     "A2" : "CSS4",
                                     "A1" : "CSS4"
                             },
                             "level" : "Amature",
                             "tags" : "CSS",
                             "question" : "CSS4"
                     },
                     {
                             "correctanswer" : {
                                     "A1" : "CSS5"
                             },
                             "answeroption" : {
                                     "A4" : "CSS5",
                                     "A3" : "CSS5",
                                     "A2" : "CSS5",
                                     "A1" : "CSS5"
                             },
                             "level" : "Amature",
                             "tags" : "CSS",
                             "question" : "CSS5"
                     },
                     {
                             "correctanswer" : {
                                     "A1" : "CSS6"
                             },
                             "answeroption" : {
                                     "A4" : "CSS6",
                                     "A3" : "CSS6",
                                     "A2" : "CSS6",
                                     "A1" : "CSS6"
                             },
                             "level" : "Amature",
                             "tags" : "CSS",
                             "question" : "CSS6"
                     },
                     {
                             "correctanswer" : {
                                     "A1" : "CSS7"
                             },
                             "answeroption" : {
                                     "A4" : "CSS7",
                                     "A3" : "CSS7",
                                     "A2" : "CSS7",
                                     "A1" : "CSS7"
                             },
                             "level" : "Amature",
                             "tags" : "CSS",
                             "question" : "CSS7"
                     },
                     {
                             "correctanswer" : {
                                     "A1" : "CSS8"
                             },
                             "answeroption" : {
                                     "A4" : "CSS8",
                                     "A3" : "CSS8",
                                     "A2" : "CSS8",
                                     "A1" : "CSS8"
                             },
                             "level" : "Amature",
                             "tags" : "CSS",
                             "question" : "CSS8"
                     },
                     {
                             "correctanswer" : {
                                     "A1" : "CSS9"
                             },
                             "answeroption" : {
                                     "A4" : "CSS9",
                                     "A3" : "CSS9",
                                     "A2" : "CSS9",
                                     "A1" : "CSS9"
                             },
                             "level" : "Amature",
                             "tags" : "CSS",
                             "question" : "CSS9"
                     },
                     {
                             "correctanswer" : {
                                     "A1" : "CSS10"
                             },
                             "answeroption" : {
                                     "A4" : "CSS10",
                                     "A3" : "CSS10",
                                     "A2" : "CSS10",
                                     "A1" : "CSS10"
                             },
                             "level" : "Amature",
                             "tags" : "CSS",
                             "question" : "CSS10"
                     }
             ]
     },
     "__v" : 3
}

从问题数组中，我希望每次触发查询时都会得到一个随机对象（随机问题）。

我不想一次收集所有对象并在节点中进行处理。是否可以编写一个查询，以便每次都返回一个随机对象？

Answer 1

试试这个逻辑

1）在数组＆＃34;问题＆＃34;上使用$unwind，如果使用includeArrayIndex，它将在展开文档中创建带索引的文档examples

2）在展开数组后，传递一个随机数来检索问题

Answer 2

您需要使用$unwind运算符对“questions”数组进行反规范化。从那里，您可以使用$sample管道运算符返回随机文档（随机问题）。

db.collection.aggregate(
    [ 
        { "$unwind": "$questionLibrary.questions" }, 
        { "$sample": { "size": 1 } } 
    ]
)

但我建议您更改文档结构并将“问题”保存到自己的集合中，因为$unwind可以产生非常大的结果。