MongoDB $elemMatch not working for subdocument

Question

Im trying to get specific quiz in database by id:

static getQuiz(db, id, cb){ //db is database connection, id is quiz id, cb is just callback
        db.find({ _id : "8RA4Rey50eqKFlWK"}, {"quiz" : { $elemMatch : { _id : id}}}, function(err, Doc){
            if(cb){
                cb(err,Doc);
                console.log(Doc); //only return _id : "8RA4Rey50eqKFlWK"
            }
        })
    }

JSON Database:

{
   "_id":"8RA4Rey50eqKFlWK",
   "quiz":[
      {
         "_id":"1b944055-2b15-4838-7e7a-beef4c9a5a62",
         "title":"test",
         "description":""
      },
      {
         "_id":"7dc53529-206c-6003-1d3c-133264d7ad81",
         "title":"aaaa",
         "description":""
      },
      {
         "_id":"db3c788f-56b3-f9c8-8a25-affb2981e12f",
         "title":"lala",
         "description":""
      },
      {
         "_id":"20388c1f-1a00-4f7b-3d25-9db56247a6bf",
         "title":"asdasd",
         "description":""
      }
   ]
}

above code not working, already tested with:

db.find({ _id : "8RA4Rey50eqKFlWK"}, {"quiz" : { $elemMatch : { title : "test"}}})

but result still just root id => _id : "8RA4Rey50eqKFlWK".

i expects result when search quiz by id "1b944055-2b15-4838-7e7a-beef4c9a5a62" should be:

{
   "_id":"8RA4Rey50eqKFlWK",
   "quiz":[
      {
         "_id":"1b944055-2b15-4838-7e7a-beef4c9a5a62",
         "title":"test",
         "description":""
      },
    ]
}

Any solutions? im actually using NeDB https://github.com/louischatriot/nedb which is same syntax as MongoDB. thanks

EDIT: i tried my code using mongodb console, its works beautifully! maybe thats just NeDB bug?

I tried your code now in mongo2.6.9 and returns the expected result. — Moi Syme, Apr 07 '17 at 18:39
Yes that looks like a functionality not provided in nedb. Have you tried positional variant ? `db.find( { _id : "8RA4Rey50eqKFlWK", "quiz" : { $elemMatch : { title : "test" } } }, { "quiz.$" : 1 } )` or `db.find( { _id : "8RA4Rey50eqKFlWK", "quiz.title" : "test" }, { "quiz.$" : 1 } )` — s7vr, Apr 07 '17 at 19:46
It looks like the issue is that you only want to return a specific element of the "quiz" array. $elemMatch is used to match based on elements of the array but will still return the entire document. See my answer below about using the aggregation framework to restructure the document. — Adam Harrison, Apr 07 '17 at 21:45

score 0 · Answer 1 · answered Apr 07 '17 at 18:05

0

        db.a.find(     
          {_id : "8RA4Rey50eqKFlWK"},
          { 
             "quiz" : { 
                  $elemMatch : { "title" : "test"}
                     }
          });

miss spelled title to _title

answered Apr 07 '17 at 18:05

Vinayk93

353
1
6

oops. i miss spelled here, but not in my actual code. sorry. – tonywei Apr 07 '17 at 18:15

score 0 · Answer 2 · answered Apr 07 '17 at 18:20

0

You can try this:

db.collection.find( { 
        _id: "8RA4Rey50eqKFlWK",
        "quiz" : { 
           $elemMatch : { "title" : "test"}
              }} );

answered Apr 07 '17 at 18:20

Moi Syme

466
2
8

tried your code, the result is all the data in database. thanks anyway – tonywei Apr 07 '17 at 18:26
btw, i tried it in mongodb console, the code works fine,maybe its just nedb specific bug. idk – tonywei Apr 07 '17 at 18:29
Really sorry !!! my fault now I read again your query. I will try again to solve the problem – Moi Syme Apr 07 '17 at 18:31

score 0 · Answer 3 · edited May 23 '17 at 11:54

If I understand correctly, you want a query to return the document in included in your message (the one with "_id":"8RA4Rey50eqKFlWK") and only display the quiz with "_id":"1b944055-2b15-4838-7e7a-beef4c9a5a62", is this correct?

If so, you'll need to use the aggregation framework.

The following aggregation framework query will query for the document by _id, unwind the "quiz" array and filter out the specific sub-document in the array:

db.quiz.aggregate([ 
{ "$match" : { _id: "8RA4Rey50eqKFlWK" } }, 
{ "$unwind" : "$quiz" },
{ "$match" : { "quiz._id" : "1b944055-2b15-4838-7e7a-beef4c9a5a62" } }
] )

Returns:

{ "_id" : "8RA4Rey50eqKFlWK", "quiz" : { "_id" : "1b944055-2b15-4838-7e7a-beef4c9a5a62", "title" : "test", "description" : "" } }

I provided a detailed breakdown of the aggregation framework in another one of my answers.

I tried with NeDB, aggregate() is not supported, but thanks anyway Mr. Harrison — tonywei, Apr 08 '17 at 02:36

MongoDB $elemMatch not working for subdocument

3 Answers3