db={"categories":[{"_id":ObjectId("61740086893f048528d166b9"),"name":"Category1","tracks":["61c65353565a2d9a1cd3020d","61c74518962dc3efb96c3438","61c74775703176a6f72df444"]}],"tracks":[{"_id":ObjectId("61c65353565a2d9a1cd3020d"),"name":"Track1","categoryId":ObjectId("61740086893f048528d166b9"),"creatorId":ObjectId("61c6478304e98ed63e8ee7d3"),"thumbnailId":ObjectId("61c65353565a2d9a1cd3020c"),"plays":[],"media":{"type":"wav","url":""},"status":"approved","downloads":[],"uploadedDate":1640387411}]}

db.categories.aggregate([{$project:{tracks:{$slice:["$tracks",-3]}}},{$project:{tracks:{"$map":{"input":"$tracks","as":"t","in":{"$toObjectId":"$$t"}}}}},{"$lookup":{"from":"tracks",let:{t:"$tracks"},pipeline:[{$match:{$expr:{"$in":["$_id","$$t"]}}}],"as":"tracks"}}])