Database
[{"_id":"600331b3d84ac418877a0e5a","tasksPerformed":["5fffb180a477c4f78ad67331","5fffb18aa477c4f78ad67332"],"year":2003,eightHourDays:2,tenHourDays:3,twelveHourDays:4},{"_id":"600331dcd84ac418877a0e5c","tasksPerformed":["5fffb180a477c4f78ad67331"],"year":2003}]
Query
db.collection.aggregate([{$group:{_id:"$year",history:{$push:"$$ROOT"},totalHoursForYear:{$sum:{$add:[{$multiply:["$eightHourDays",8]},{$multiply:["$tenHourDays",10]},{$multiply:["$twelveHourDays",12]},]}},}}])