update nested array object using positional operator

Question

my document object looks like this

{ 
   "_id" : ObjectId("5b62cfac2046e98de373399c"),
   "hindi" : {
    "confirmed" : false, 
    "assigned_to" : "[email protected]", 
    "candidates" : [

            {
            "candidate_id" : "9262c520-9640-11e8-bfbf-292ac77d55f9", 
            "value" : "स्क्रॉल कैसे करें ?", 
            "createdAt" : NumberInt(1533207154), 
            "createdBy" : "[email protected]", 
            "warningList" : [], 
            "comments" : [
                "Correction कैसे स्क्रॉल करें", 
                "added ?"
            ]
           }, 
           {
            "candidate_id" : "297bb060-9642-11e8-ac2d-93ac27f5ee90", 
            "value" : "स्क्रॉल कैसे करें ?", 
            "createdAt" : NumberInt(1533207154), 
            "createdBy" : "[email protected]", 
            "warningList" : [], 
            "comments" : [
                "Correction कैसे स्क्रॉल करें", 
                "added ?"
            ]
          }
    ]
}

My objective is to find an particular item inside candidates array by the candidate_id and update the comment field.

Here is the query I'm trying with MongoDB positional operator($) but getting an error Unsupported projection option: $set: { hindi.candidates.$.comments: [] }

db.getCollection("strings").update({_id: ObjectId("5b62cfac2046e98de373399c"),"hindi.candidates.candidate_id": "297bb060-9642-11e8-ac2d-93ac27f5ee90"}, {$set :{"hindi.candidates.$.comments" : []}})

Do you want to push something in commentsarray or want to $set it as []? — Ashh
– Ashh, Commented Aug 2, 2018 at 18:41
Correct syntax is db.getCollection("strings").update( { "_id": ObjectId("5b62cfac2046e98de373399c"), "hindi.candidates": { "$elemMatch": { "candidate_id": "297bb060-9642-11e8-ac2d-93ac27f5ee90" } } }, { "$set": { "hindi.candidates.$.comments" : [] }} ) — Ashh
– Ashh, Commented Aug 2, 2018 at 18:47
@AnthonyWinzlet it depends on the requrement, sometimes just needs to be set as empty, sometimes it can contain new values — izengod
– izengod, Commented Aug 2, 2018 at 18:56
you have used incorrect syntax for update query... Try the above one — Ashh
– Ashh, Commented Aug 2, 2018 at 18:59
@AnthonyWinzlet working fine now. could you please elaborate what was the mistakes? — izengod
– izengod, Commented Aug 2, 2018 at 19:04

Ashh · Accepted Answer · 2018-08-02 19:07:53Z

1

You need to use $elemMatch here to match inside an array

db.getCollection("strings").update(
  {
    "_id": ObjectId("5b62cfac2046e98de373399c"),
    "hindi.candidates": { "$elemMatch": { "candidate_id": "297bb060-9642-11e8-ac2d-93ac27f5ee90" } }
  }, 
  { "$set": { "hindi.candidates.$.comments" : [] }}
)

answered Aug 2, 2018 at 19:07

Ashh

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