Get random element from array in MongoDB

Question

Here is my database structure

{
     "_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
}

From the question array, I want to get a random object (random question) each time I fire the query.

I don't want to collect all the objects at a time and manupulate them in node. Is it possible to write a query so that it will return a random object each time?

Answer 1

Try this logic

1) use $unwind on the array "questions", if you use includeArrayIndex, it will create documents with index see examples in unwind documentation

2) After unwinding the array pass a random number to retrieve a question

Answer 2

You need to de-normalize the "questions" array using the $unwind operator. From there you can use the $sample pipeline operator to return a random document (random question).

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

But I suggest you change your document structure and save "questions" into it own collection because $unwind can produce a very large result.