db={products:[{"_id":ObjectId("63513c705f31b4bcb75b80ce"),"name":"Coca-cola","stocks":[ObjectId("63513c705f31b4bcb75b80d0"),ObjectId("63513c705f31b4bcb75b80d1")]}],stock:[{"_id":ObjectId("63513c705f31b4bcb75b80d0"),"count":9,"remaining":6,"costPerItem":10,"createdAt":ISODate("2022-10-20T12:17:52.985Z"),},{"_id":ObjectId("63513c705f31b4bcb75b80d1"),"count":10,"remaining":3,"costPerItem":10,"createdAt":ISODate("2022-10-20T12:17:52.985Z"),}]}

db.products.aggregate([{$lookup:{from:"stock",let:{stocks:"$stocks"},pipeline:[{$match:{$expr:{$in:["$_id","$$stocks"]}}},{$project:{_id:0,remaining:1}}],as:"remaining"}},{$set:{remaining:{$sum:"$remaining.remaining"}}},{$match:{remaining:{$lt:100}}}])