Returning only sub document based on subdocument _id which is the items of array field of the mongodb document

Question

My schema goes like this

  [
{
    "_id" : ObjectId("565f0f5d77f0c7bd11bbadd8"),
    "blog_id" : ObjectId("56587befdb7224110f007233"),
    "comments" : [
        {
            "user_id" : ObjectId("562fa014888806820e21e0df"),
            "user_full_name" : "Niroj Paudel",
            "comment" : "pradip is bhole baba",
            "_id" : ObjectId("565f0f5d77f0c7bd11bbadd9"),
            "dt" : ISODate("2015-12-02T15:33:49.578Z")
        },
        {
            "user_id" : ObjectId("562fa014888806820e21e0df"),
            "user_full_name" : "Niroj Paudel",
            "comment" : "honkog pokhara... he he ha ha",
            "_id" : ObjectId("565f1034fd07cbfc1129db0b"),
            "dt" : ISODate("2015-12-02T15:37:24.581Z")
        }
    ],
    "record_count" : 2,
    "__v" : 0
}]

What I am trying is to select particular item of comments array based on the blog_id and comments._id ..... but instead of returning only the particular comment it is returning the whole document .

Currently I have following query

db.blog_comments..findOne({
           "blog_id" : ObjectId("56587befdb7224110f007233"),
           "comments._id":ObjectId("565f1034fd07cbfc1129db0b")
       })

This query is returning the whole document i.e.-

[
{
    "_id" : ObjectId("565f0f5d77f0c7bd11bbadd8"),
    "blog_id" : ObjectId("56587befdb7224110f007233"),
    "comments" : [
        {
            "user_id" : ObjectId("562fa014888806820e21e0df"),
            "user_full_name" : "Niroj Paudel",
            "comment" : "pradip is bhole baba",
            "_id" : ObjectId("565f0f5d77f0c7bd11bbadd9"),
            "dt" : ISODate("2015-12-02T15:33:49.578Z")
        },
        {
            "user_id" : ObjectId("562fa014888806820e21e0df"),
            "user_full_name" : "Niroj Paudel",
            "comment" : "honkog pokhara... he he ha ha",
            "_id" : ObjectId("565f1034fd07cbfc1129db0b"),
            "dt" : ISODate("2015-12-02T15:37:24.581Z")
        }
    ],
    "record_count" : 2,
    "__v" : 0
}]

but I want just this

[{
                "user_id" : ObjectId("562fa014888806820e21e0df"),
                "user_full_name" : "Niroj Paudel",
                "comment" : "honkog pokhara... he he ha ha",
                "_id" : ObjectId("565f1034fd07cbfc1129db0b"),
                "dt" : ISODate("2015-12-02T15:37:24.581Z")
 }]

What I am missing please suggest.. Thank you

Answer 1

you can use $elemMatch in your projection:

test> db.project_sub.findOne({
...    "blog_id" : ObjectId("56587befdb7224110f007233")
... },{
...    "comments": { $elemMatch: { _id: ObjectId("565f1034fd07cbfc1129db0b") } }
... })
{
  "_id": ObjectId("565f0f5d77f0c7bd11bbadd8"),
  "comments": [
    {
      "user_id": ObjectId("562fa014888806820e21e0df"),
      "user_full_name": "Niroj Paudel",
      "comment": "honkog pokhara... he he ha ha",
      "_id": ObjectId("565f1034fd07cbfc1129db0b"),
      "dt": ISODate("2015-12-02T15:37:24.581Z")
    }
  ]
}