[{"_id":NumberInt(1),"absences":170.0,"dayNumber":NumberInt(1),"dayName":"Sunday"},{"_id":NumberInt(2),"absences":130.0,"dayNumber":NumberInt(2),"dayName":"Monday"}]

db.collection.aggregate([{$group:{_id:null,total:{$sum:"$absences"},items:{$push:"$$ROOT"}}},{$unwind:"$items"},{"$replaceRoot":{"newRoot":{$mergeObjects:[{total:"$total"},"$items"]}}}])