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 an array of questions, I want to get a random object (random question) every time I run a query.

I do not want to collect all the objects at a time and create them in node. Is it possible to write a query so that it returns a random object every time?

Answer 1

Try this logic

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

2) After unwinding, the array passes a random number to get the question

Answer 2

You need to de-normalize the array of questions 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 the structure of the document and keep the “questions” in your own collection, because $unwind can produce a very big result.