Question

Is that possible to sort data in sub array in mongo database?

{   "_id" : ObjectId("4e3f8c7de7c7914b87d2e0eb"), 
    "list" : [
            {
                "id" : ObjectId("4e3f8d0be62883f70c00031c"),
                "datetime" : 1312787723,
                "comments" : 
                    {
                        "id" : ObjectId("4e3f8d0be62883f70c00031d")
                        "datetime": 1312787723,
                    },
                    {
                        "id" : ObjectId("4e3f8d0be62883f70c00031d")
                        "datetime": 1312787724,
                    },                  
                    {
                        "id" : ObjectId("4e3f8d0be62883f70c00031d")
                        "datetime": 1312787725,
                    },
            }
        ], 
    "user_id" : "3" }

For example I want to sort comments by field “datetime”. Thanks. Or only variant is to select all data and sort it in PHP code, but my query works with limit from mongo…

Answer 1

With MongoDB, you can sort the documents or select only some parts of the documents, but you can’t modify the documents returned by a search query.

If the current order of your comments can be changed, then the best solution would be to sort them in the MongoDB documents (find(), then for each doc, sort its comments and update()). If you want to keep the current internal order of comments, then you’ll have to sort each document after each query.

In both case, the sort will be done with PHP. Something like:

foreach ($doc['list'] as $list) {
    // uses a lambda function, PHP 5.3 required
    usort($list['comments'], function($a,$b){ return $a["datetime"] < $b["datetime"] ? -1 : 1; });
}

If you can’t use PHP 5.3, replace the lambda function by a normal one. See usort() examples.