MongoDB query, project nested docs?

Question

I was wondering if this is possible, or do I need to use the aggregation pipeline instead?

I have read posts such as this, and intuitively feel like it's possible.

Example of docs:

{
    "_id": ObjectID("5143ddf3bcf1bfab37d9c6f"),
    "permalink": "btxcacmbqkxbpgtpeero",
    "author": "machine",
    "title": "Declaration of Independence",
    "tags": [
            "study",
            "law"
    ],
    "comments": [
                  {
                    "body": "comment 1",
                    "email": "[email protected]",
                    "author": "machine_1"
                   },
                  {
                    "body": "comment 2",
                    "email": "[email protected]",
                    "author": "machine_2"
                   },
                  {
                    "body": "comment 3",
                    "email": "[email protected]",
                    "author": "machine_3"
                   },
    ]
    "date": ISODate("2013-03-16T02:50:27.878Z")
}

I am trying to access a particular comment in "comments" by it's index position using dot notation in the projection field, with the following:

db.collection.find({permalink: "btxcacmbqkxbpgtpeero"}, {'comments.0.1.': 1})

Where comments.0 is the first item in the field: the array, and .1 is the second comment in the array.

The result I am getting:

{ "_id" : ObjectID("5143ddf3bcf1bfab37d9c6f"), "comments" : [ {  }, {  }, {  } ] }

If I take away the .1, leaving just comments.0, I get the same result:

{ "_id" : ObjectID("5143ddf3bcf1bfab37d9c6f"), "comments" : [ {  }, {  }, {  } ] }

If I take away the .0, leaving just comments, I get the comments still inside their array:

[
   {
    "body": "comment 1",
    "email": "[email protected]",
    "author": "machine_1"
   },
   {
     "body": "comment 2",
     "email": "[email protected]",
     "author": "machine_2"
   },
   {
     "body": "comment 3",
     "email": "[email protected]",
     "author": "machine_3"
   }
]

Can this be done? If so, how?

Possible duplicate of MongoDB : use $ positional operator for querying — Neo-coder
– Neo-coder, Commented Nov 26, 2016 at 17:23
@Yogesh Not quite; that post is focused on using the $ operator. I am trying to explicitly specify the position with the index of the element. "The positional $ operator identifies an element in an array to update without explicitly specifying the position of the element in the array". Also I'm querying, and using this in the projection field, whereas the docs seem to say the $ operator is for updates. — Jake Stokes
– Jake Stokes, Commented Nov 27, 2016 at 14:28

felix · Accepted Answer · 2019-04-03 08:21:29Z

without aggregation:

db.collection.find({
   permalink:"btxcacmbqkxbpgtpeero"
},
{
   comments:{
      $slice:[
         0,
         1
      ]
   }
})

returns

{
   "_id":ObjectId("583af24824168f5cc566e1e9"),
   "permalink":"btxcacmbqkxbpgtpeero",
   "author":"machine",
   "title":"Declaration of Independence",
   "tags":[
      "study",
      "law"
   ],
   "comments":[
      {
         "body":"comment 1",
         "email":"[email protected]",
         "author":"machine_1"
      }
   ]
}

try it online: mongoplayground.net/p/LGbVWPyVkFk

with aggregation:

db.collection.aggregate([
   {
      $match:{
         permalink:"btxcacmbqkxbpgtpeero"
      }
   },
   {
      $project:{
         comment:{
            $arrayElemAt:[
               "$comments",
               0
            ]
         }
      }
   }
])

returns

{
   "_id":ObjectId("583af24824168f5cc566e1e9"),
   "comment":{
      "body":"comment 1",
      "email":"[email protected]",
      "author":"machine_1"
   }
}

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