Removing elements from array of array : MongoDB

Question

{
    "_id" : "TestData123",
    "data" : [ 
        {
            "ch" : "test1",
            "prod" : [ 
                {
                    "sid" : "1",
                    "value" : " TV"
                }, 
                {
                    "sid" : "2",
                    "value" : "amazon",
                }, 
                {
                    "sid" : "3",
                    "value" : "ebay",
                }, 
                {
                    "sid" : "4",
                    "value" : "otherthing",
                }
            ]
        }, 
        {
            "ch" : "test2",
            "prod" : [ 
                {
                    "sid" : "6",
                    "value" : "TV",
                }, 
                {
                    "sid" : "7",
                    "value" : "amazon",
                }, 
                {
                    "sid" : "8",
                    "value" : "ebay",
                }, 
                {
                    "sid" : "9",
                    "value" : "otherthing",
                }
            ]
        }
    ]
}

I have a mongoDB collection in the above format. I need to remove/empty the array 'prod' in ch:test1 and ch:test2. Also please suggest the mongo query for adding the elements to these as well.

You want to remove some element from the array or empty the array? — Indrajeet
– Indrajeet, Commented Apr 7, 2015 at 17:46

Community · Accepted Answer · 2017-05-23 12:06:33Z

I've cleaned up and formatted your BSON object:

{
   "_id":"TestData123",
   "data":[
      {
         "ch":"test1",
         "prod":[
            {
               "sid":"1",
               "value":" TV"
            },
            {
               "sid":"2",
               "value":"amazon"
            },
            {
               "sid":"3",
               "value":"ebay"
            },
            {
               "sid":"4",
               "value":"otherthing"
            }
         ]
      },
      {
         "ch":"test2",
         "prod":[
            {
               "sid":"6",
               "value":"TV"
            },
            {
               "sid":"7",
               "value":"amazon"
            },
            {
               "sid":"8",
               "value":"ebay"
            },
            {
               "sid":"9",
               "value":"otherthing"
            }
         ]
      }
   ]
}

Here's another stackoverflow post with the same question:

https://stackoverflow.com/a/6327971/980423

You're looking for the $unset operator.

  db.tmp.update({"_id":"TestData123", "data.ch": "test1"}, {$unset: {"data.$.prod": 1}});

It's also tricky because it's in an array.

  db.tmp.update({"_id":"TestData123", "data.ch": "test1"}, {$set: {"data.$.prod": <some array>}});

Will set it again.

ahebib · Accepted Answer · 2015-04-07 18:02:49Z

I'm not sure what language you're using but here is how I would do it in C#. I would first get the document with the id.

var dbcollection = db.GetCollection("");
var document = dbcollection.FindOneById(ObjectId.Parse(id));
var query = new QueryDocument { { "_id", ObjectId.Parse(id) } };

Then I would update the document with the modified version.

BsonArray array = new BsonArray(); //leave empty or add same structure but empty fields
UpdateBuilder update = new UpdateBuilder();
update.Set("data.0.prod", array );
var result = dbcollection.Update(query, update);

This will empty the array. I haven't emptied an array, I've just updated one but I think this will work.

This also might work I haven't tested it out yet.

dbcollection.Remove(Query.EQ("_id", ObjectId.Parse(id)), document["data"][0]["prod"] );

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