update nth document in a nested array document in mongodb

Question

I need to update a document in an array inside another document in Mongo DB

    {
            "_id" : ObjectId("51cff693d342704b5047e6d8"),
            "author" : "test",
            "body" : "sdfkj dsfhk asdfjad ",
            "comments" : [
                    {
                            "author" : "test",
                            "body" : "sdfkjdj\r\nasdjgkfdfj",
                            "email" : "[email protected]"
                    },
                    {
                            "author" : "hola",
                            "body" : "sdfl\r\nhola \r\nwork here"
                    }
            ],
            "date" : ISODate("2013-06-30T09:12:51.629Z"),
            "permalink" : "jaiho",
            "tags" : [
                    "jaiho"
            ],
            "title" : "JAiHo"
    }


Q1) Update email of 0th element of comments array
db.posts.update({"permalink" : "haha"},{$set:{"comments.0.email":1}})
This doesn't throw any exception but doesn't update anything as well
Q2) Add a field on nth element of comments array number_likes
db.posts.update({"permalink" : "haha"},{$inc:{"comments.0.num_likes":1}})
Doesn't work either.

Am I missing something here?

Lol, this is almost certainly from the 10gen mongo course :P education.10gen.com/courses — Mike Graf
– Mike Graf, Commented Aug 1, 2013 at 14:41
Yeah actually, but couldn't find what went wrong with the solution and didn't wanted to fail the test ;) You may look at it as cheating but may be I'm still learning. — Amit Sehgal
– Amit Sehgal, Commented Aug 13, 2013 at 16:29

Joachim Isaksson · Accepted Answer · 2013-06-30 10:22:47Z

8

Q1: If you update with permalink 'jaiho' instead of 'haha', it most certainly updates the email;

> db.posts.update({"permalink" : "jaiho"},{$set:{"comments.0.email":1}})
> db.posts.find()
    ...,    "email" : 1 },...

Q2: Same goes for this include;

> db.posts.update({"permalink" : "jaiho"},{$inc:{"comments.0.num_likes":1}})
> db.posts.find()
    ..., "num_likes" : 1 },...

answered Jun 30, 2013 at 10:22

Joachim Isaksson

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rohit hiresheddi · Accepted Answer · 2013-10-01 12:17:10Z

6

If you are trying to do it dynamically in Node JS following should work.

i = 0;
selector = {};
operator = {};
selector['comments.' + i + '.email'] = 1; // {'comments.0.num_likes' : 1}
operator['$inc'] = selector;  // {'$inc' : {'comments.0.num_likes' : 1} }
db.posts.update({'permalink' : 'xyz'}, operator);

answered Oct 1, 2013 at 12:17

rohit hiresheddi

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