Get matching elements in mongodb array

Question

I want to use aggregation to get this array only with those tickets, which have start field after 2015-06-16. Can someone help me with the pipeline?

{
    "name" : "array",
    "tickets" : [ 
        {
            "id" : 1,
            "sort" : true,
            "start" : ISODate("2015-06-15T22:00:00.000Z")
        },
        {
            "id" : 2,
            "sort" : true,
            "start" : ISODate("2015-06-16T22:00:00.000Z")
        },
        {
            "id" : 3,
            "sort" : true,
            "start" : ISODate("2015-06-17T22:00:00.000Z")
        }
    ]
}

Answer 1

It's true that the "standard projection" operations available to MongoDB methods such as .find() will only return at most a "single matching element" from the array to that is queried by either the positional $ operator form in the "query" portion or the $elemMatch in the "projection" portion.

In order to do this sort of "ranged" operation, you need the aggregation framework which has greater "manipulation" and "filtering" capabilities on arrays:

 collection.aggregate(
     array( 
         # First match the "document" to reduce the pipeline
         array(
             '$match' => array(
                 array( 
                    'tickets.start' => array(
                        '$gte' => new MongoDate(strtotime('2015-06-16 00:00:00'))
                    )
                )
             )
         ),

         # Then unwind the array
         array( '$unwind' => '$tickets' ),

         # Match again on the "unwound" elements to filter
         array(
             '$match' => array(
                 array( 
                    'tickets.start' => array(
                        '$gte' => new MongoDate(strtotime('2015-06-16 00:00:00'))
                    )
                )
             )
         ),

         # Group back to original structure per document
         array(
             '$group' => array( 
                  '_id' => '$_id',
                  'name' => array( '$first' => '$name' ),
                  'tickets' => array(
                      '$push' => '$tickets'
                  )
             )
         )           
     )
 ) 

Or you can possibly use the $redact operator to simplify with MongoDB 2.6 or greater which basically uses the $cond operator syntax as it's input:

 collection.aggregate(
     array( 
         # First match the "document" to reduce the pipeline
         array(
             '$match' => array(
                 array( 
                    'tickets.start' => array(
                        '$gte' => new MongoDate(strtotime('2015-06-16 00:00:00'))
                    )
                )
             )
         ),

         # Redact entries from the array
         array(
             '$redact' => array(
                 'if' => array(
                     '$gte' => array(
                         array( '$ifNull' => array(
                             '$start',
                             new MongoDate(strtotime('2015-06-16 00:00:00'))
                         )),
                         new MongoDate(strtotime('2015-06-16 00:00:00:00'))
                     )
                 ),
                 'then' => '$$DESCEND',
                 'else' => '$$PRUNE'
             )
         )
     )
 ) 

So both examples do the "same thing" in "filtering" the elements from the array that "do not" match the conditions specified and return "more than one" element, which is something basic projection cannot do.

Answer 2

You should use Aggregation to get output.

You should use following query:

db.collection.aggregate({
    $match: {
    name: "array"
    }
}, {
    $unwind: "$tickets"
}, {
    $match: {
    "tickets.start": {
        $gt: ISODate("2015-06-16")
    }
    }
}, {
    $group: {
    "_id": "name",
    "tickets": {
        $push: "$tickets"
    }
    }
})