Why MongoDB $elemMatch returns all elements in array?

Question

In mongoDB (2.4) I have collection students with bunch of documents in it. Each document will have structure as below:

{
    "_id" : ObjectId("55f16640a17eed480bb0dc18"),
    "messages" : [
        {
            "_id" : ObjectId("55fc60bc260c9be5879ef733"),
            "createdOn" : ISODate("2015-09-18T19:06:36.793Z"),
            "fromUser" : ObjectId("55f16a6af9c703280c5b2c0e"),
            "toUser" : ObjectId("55f16640a17eed480bb0dc18"),
            "type" : 1
        },
        {
            "_id" : ObjectId("55fc6d200d6097e98b210bff"),
            "createdOn" : ISODate("2015-09-18T19:59:28.439Z"),
            "fromUser" : ObjectId("55f721d1e1242e7c4c129471"),
            "toUser" : ObjectId("55f16640a17eed480bb0dc18"),
            "type" : 1
        }
    ]
}

Now, I need to pull out one of the subdocuments from messages array, say with messages._id equal 55fc60bc260c9be5879ef733 so I use $elemMatch:

db.users.find({"_id": new ObjectId('55f16640a17eed480bb0dc18'),
                "messages": {
                    $elemMatch:{
                        "_id": new ObjectId('55fc60bc260c9be5879ef733'),
                        "fromUser": new ObjectId('55f16a6af9c703280c5b2c0e'),
                        "toUser": new ObjectId('55f16640a17eed480bb0dc18'),
                        "type": 1

                }}}).pretty();

But this gives me:

{
    "_id" : ObjectId("55f16640a17eed480bb0dc18"),
    "messages" : [
        {
            "_id" : ObjectId("55fc60bc260c9be5879ef733"),
            "createdOn" : ISODate("2015-09-18T19:06:36.793Z"),
            "fromUser" : ObjectId("55f16a6af9c703280c5b2c0e"),
            "toUser" : ObjectId("55f16640a17eed480bb0dc18"),
            "type" : 1
        },
        {
            "_id" : ObjectId("55fc6d200d6097e98b210bff"),
            "createdOn" : ISODate("2015-09-18T19:59:28.439Z"),
            "fromUser" : ObjectId("55f721d1e1242e7c4c129471"),
            "toUser" : ObjectId("55f16640a17eed480bb0dc18"),
            "type" : 1
        }
    ]
}

If I understand docs correctly http://docs.mongodb.org/v2.4/reference/operator/projection/elemMatch/ I should receive only:

{
    "_id" : ObjectId("55f16640a17eed480bb0dc18"),
    "messages" : [
        {
            "_id" : ObjectId("55fc60bc260c9be5879ef733"),
            "createdOn" : ISODate("2015-09-18T19:06:36.793Z"),
            "fromUser" : ObjectId("55f16a6af9c703280c5b2c0e"),
            "toUser" : ObjectId("55f16640a17eed480bb0dc18"),
            "type" : 1
        }
    ]
}

So what am I doing wrong?

Answer 1

You are not using the projection operator $elemMatch match, but the query operator $elemMatch

In other words, you don't project anything, but use $elemMatch for filtering the results that your query should return.

In order for your projection to work, you should change your query to something like:

db.users.find(
    {"_id": new ObjectId('55f16640a17eed480bb0dc18')},
    {"messages": {$elemMatch:{"_id": new ObjectId('55fc60bc260c9be5879ef733')}}}
).pretty();

The above query should return the _id field (same as appears in the query clause), and the array element whose id is mentioned in the projection clause