db={"orders":[{"_id":1,"shop":1,"price":11},{"_id":2,"shop":2,"price":101},],"shops":[{"_id":1,},{"_id":2,},{"_id":3,},],}

db.shops.aggregate([{$lookup:{from:"orders",let:{shop:"$_id"},pipeline:[{$match:{$expr:{$eq:["$shop","$$shop"]},},},],as:"orders",},},{$project:{user:"$user",orderCnt:{$size:"$orders"}}},{$match:{orderCnt:{$eq:0}}},])