Mongodb - use of $ , updating a value in a sub-array

Question

I have a collection (favorites) with documents that look like this:

{
    "_id" : 907,
    "pictures" : [
        {
        "id" : 107,
        "url" : "http://url1",
        "title" : "some title"
        },
        {
        "id" : 111,
        "url" : "http://url2",
        "title" : "some other title"
        }
    ]
}

It's fairly easy to get just the urls using pictures.url.

But how do I update the url for all users that saved the picture with an id=111 as their favorite?

I found an work-around using:

107:['id':107,'url':'http://url1','title':'some title']

...and then using find() with pictures.107.id => 107, but that looks silly to me.

Is there a better way to achieve this?

Which URL do you want to update? The one for 107 or 111? Or both? — netcoder
– netcoder, Commented Mar 27, 2012 at 13:29
doesn't matter , really, I am just looking to see if there's a better way of doing multiple updates, without relying on the workaround I mentioned (basically storing the picture_id twice), and hopefully still preserving the pictures.url in find(). with the workaround I am forced to get from mongo all fields in pictures — Johny K.
– Johny K., Commented Mar 27, 2012 at 13:33
I posted an answer but given the other replies I may not have understood your original question (which, frankly, is a little unclear). What exactly is it you need to do? Just update all "url" values for all users that have 111 as their favorite? — Remon van Vliet
– Remon van Vliet, Commented Mar 27, 2012 at 14:22

netcoder · Accepted Answer · 2012-03-27 14:19:52Z

3

Not entirely sure what you're looking for, but I'll give it a shot.

In MongoDB, you can do atomic updates with the $set operator, which will replace only the data you pass and not the whole document.

If you use it with the $elemMatch operator when selecting, you can update only the url of the pictures that match your query:

$mongo->selectCollection('mydb', 'favorites')
      ->update(
           array('pictures' => array(
               '$elemMatch' => array('id' => 107),
           )),
           array('$set' => array('pictures.$.url' => 'http://foobar'))
        );

You can see that the update contains pictures.$.url, where the $ refers to the element(s) that matched the $elemMatch query.

The above will update all the pictures.url in your favorites that match 111 as their pictures.id, giving you something like:

{ "_id" : 907, "pictures" : [
    {
        "id" : 107,
        "url" : "http://url1",
        "title" : "some title"
    },
    {
        "id" : 111,
        "title" : "some other title",
        "url" : "http://foobar" // only this one has matched
    }
] }

answered Mar 27, 2012 at 14:19

netcoder

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1 Comment

Johny K. Over a year ago

thanks, as it turns out, the solution was very simple - ($elemMatch)

Remon van Vliet · Accepted Answer · 2012-03-27 14:25:21Z

1

db.favorites.update({pictures: {$elemMatch:{id:111}}}, {'pictures.$.title':"Some Title"}, false, true)

edited Mar 27, 2012 at 14:25

answered Mar 27, 2012 at 14:20

Remon van Vliet

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