MongoDB telt items in array

U moet $unwind de array "sizes" en vervolgens $group door "item" en "sizes" en gebruik de $sum om de telling terug te geven.

db.collection.aggregate(
    [ 
        { "$unwind": "$sizes" }, 
        { "$group": { 
            "_id": { 
                "item": "$item", 
                "sizes": "$sizes" 
            }, 
            "count": { "$sum": 1 } 
        }}
    ]
) 

die produceert:

{ "_id" : { "item" : "EFG", "sizes" : "S" }, "count" : 1 }
{ "_id" : { "item" : "ABC", "sizes" : "L" }, "count" : 1 }
{ "_id" : { "item" : "ABC", "sizes" : "M" }, "count" : 2 }
{ "_id" : { "item" : "ABC", "sizes" : "S" }, "count" : 1 }