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

db.collection.aggregate([{$project:{openOrders:{$filter:{input:"$orders",cond:{$eq:["$$this.status","open"]}}}}},{$project:{"openOrders._id":1}}])