Join and aggregate mongo db query

mongodbquery

db.getCollection("LOG_Android_Voided_Purchases").aggregate(
    [
        { 
            "$match" : { 
                "voidedTime" : { 
                    "$gte" : ISODate("2020-06-10T00:00:00.000+0000")
                }
            }
        }, 
        { 
            "$project" : { 
                "productId" : 1.0
            }
        },
        {
            "$lookup": {
                'from'         : 'Product_Identifiers',
                'localField'   : 'productId',
                'foreignField' : 'identifier',
                'as'           : 'productData'
            }
        },
        {
            "$unwind": "$productData"
        },
        {
             "$addFields" : {
                 "amount" : {
                     "$cond" : [
                         {"$eq" : ["$productData.feature" , "Coins"]},
                         {"$toInt" :   ["$productData.amount"]},
                         0
                     ]
                 }
             }   
        },
        {
            "$group" : {
                "_id" : null,
                "sum" : {"$sum": "$amount"}
            }
        }
        
    ], 
    { 
        "allowDiskUse" : false
    }
);