how to remove item in nested array on mongo

Question

I need to remove item in nested array, following is my document

{
    "_id" : ObjectId("58760d8caa30c585ef8d3beb"),
    "results" : [ 
        {
            "item" : "A",
            "score" : 5.0,
            "answers" : [ 
                {
                    "q" : 1.0,
                    "a" : 4.0
                }, 
                {
                    "q" : 1.0,
                    "a" : 5.0
                }, 
                {
                    "q" : 2.0,
                    "a" : 6.0
                }
            ]
        }, 
        {
            "item" : "B",
            "score" : 8.0,
            "answers" : [ 
                {
                    "q" : 1.0,
                    "a" : 7.0
                }, 
                {
                    "q" : 1.0,
                    "a" : 5.0
                }, 
                {
                    "q" : 2.0,
                    "a" : 9.0
                }
            ]
        }
    ]
}

I want to remove all item in answers which q equal 1.0, following is expect document:

{
    "_id" : ObjectId("58760d8caa30c585ef8d3beb"),
    "results" : [ 
        {
            "item" : "A",
            "score" : 5.0,
            "answers" : [ 
                {
                    "q" : 2.0,
                    "a" : 6.0
                }
            ]
        }, 
        {
            "item" : "B",
            "score" : 8.0,
            "answers" : [ 
                {
                    "q" : 2.0,
                    "a" : 9.0
                }
            ]
        }
    ]
}

I used:

db.getCollection('test').update({"results.answers.q":1},
                                { $pull: {"results.$.answers": {q:1} } },
                                { multi: true })

But got:

{
    "_id" : ObjectId("58760d8caa30c585ef8d3beb"),
    "results" : [ 
        {
            "item" : "A",
            "score" : 5.0,
            "answers" : [ 
                {
                    "q" : 2.0,
                    "a" : 6.0
                }
            ]
        }, 
        {
            "item" : "B",
            "score" : 8.0,
            "answers" : [ 
                {
                    "q" : 1.0,
                    "a" : 7.0
                }, 
                {
                    "q" : 1.0,
                    "a" : 5.0
                }, 
                {
                    "q" : 2.0,
                    "a" : 9.0
                }
            ]
        }
    ]
}

in item B there still have embed documents which q equal 1

How can I do?

@SagarReddy `$pullAll` operator requires exact match on the whole object, so I don't believe it can help in this case. — Alex Goncharenko, Jan 11 '17 at 12:10

score 1 · Accepted Answer · answered Jan 11 '17 at 11:58

Unfortunately, there is no way to achieve this result with a single query, unless someone can think of something really smart with $where operator. Usage of positional operator implies that you can only target a single array element (in your case it is a single results element), and this usage is a must in any subarray pulls.

An ugly workaround here is to perform this update N times where N is the length of results array. This approach will do the job, but obviously it will be a huge performance issue if these arrays are large.

score 0 · Answer 2 · answered Jan 11 '17 at 12:10

0

using the below query will work for you. For more info regarding $pull please visit https://docs.mongodb.com/manual/reference/operator/update/pull/

db.getCollection('test').update({},{ $pull: {"results":{"answers": {$elemMatch:{q:1} }} }},{ multi: true });

answered Jan 11 '17 at 12:10

kaushik

312
1
7

1

Your update removes the whole `results` elements instead of particular `answers` ones, hence it does not satisfy the output requirements specified by the author. – Alex Goncharenko Jan 11 '17 at 12:13
There is also no need in `$elemMatch` operator when dealing with a single query condition. You could do something like this to achieve the same result: `$pull: { "results" : { "answers.q" : 1 }}` – Alex Goncharenko Jan 11 '17 at 12:28
1

Yes, @Imperator is right. – leo Jan 12 '17 at 02:35

score 0 · Answer 3 · answered Sep 24 '17 at 10:07

0

You can now achieve this with $pull, as per the MongoDB docs: https://docs.mongodb.com/manual/reference/operator/update/pull/#pull-array-of-documents

answered Sep 24 '17 at 10:07

Vaiden

15,728
7
61
91

how to remove item in nested array on mongo

3 Answers3