how to project array element in document at specific position

Use $slice to project number from array.

Query:

db.collection.find({},
{
  "phone.cell": {
    $slice: 1
  },
  "phone.home": 0
})

Result:

{
    "_id": ObjectId("5c6cc08a568f4cdf7870b3a7"),
    "phone": {
      "cell": [
        "854-6574-545"
      ]
    }
  }

Query 2:

db.collection.find({},
    {
      "_id": 0,
      "phone.cell": {
        $slice: 1
      },
      "phone.home": 0
    })

Result 2: {

    "phone": {
      "cell": [
        "854-6574-545"
      ]
    }
  }

** Final Query - using aggregate**

db.collections.aggregate([{'$match':{'phone.cell':{'$exists':true}}},
{'$project':{'_id':1,'phone.cell':{$slice:['$phone.cell',1,1]}}}])

** Output **

{
    "_id" : ObjectId("5c6cc08a568f4cdf7870b3a7"),
    "phone" : {
        "cell" : [ 
            "545-6456-545"
        ]
    }
}