[{"provider":"provider 1","debts":[{"debt":500,"paid":true},{"debt":900,"paid":false},{"debt":600,"paid":false}]},{"provider":"provider 2","debts":[{"debt":500,"paid":false}]}]

db.collection.aggregate([{$unwind:"$debts"},{$group:{_id:"$_id",total:{$sum:{$cond:[{$eq:["$debts.paid",false]},"$debts.debt",0]}}}},{$group:{_id:null,debt_providers:{$push:"$$ROOT"},sum_total_objects:{$sum:"$total"}}}])