[{"orders":[{_id:123,status:"open"},{_id:456,status:"no"}]},{"orders":[{_id:789,status:"open"},{_id:98,status:"open"}]}]

db.collection.aggregate([{$project:{openOrders:{$reduce:{input:"$orders",initialValue:[],in:{$cond:[{$eq:["$$this.status","open"]},{$concatArrays:["$$value",["$$this._id"]]},"$$value"]}}}}}])