db={"categories":[{"_id":ObjectId("61740086893f048528d166b9"),"name":"Category1","tracks":["61c65353565a2d9a1cd3020d","61c65353565a2d9a1cd30207"]},{"_id":ObjectId("61740086893f048528d166ee"),"name":"Category2","tracks":[]}],"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},{"_id":ObjectId("61c65353565a2d9a1cd30207"),"name":"Track2","categoryId":ObjectId("61740086893f048528d166b9"),"creatorId":ObjectId("61c6478304e98ed63e8ee7cb"),"thumbnailId":ObjectId("61c65353565a2d9a1cd30205"),"plays":[],"media":{"type":"wav","url":""},"status":"approved","downloads":[],"uploadedDate":1640387411}],"users":[{"_id":ObjectId("61c6478304e98ed63e8ee7d3"),"email":"user123@gmail.com","username":"user123","tracks":[ObjectId("61c65353565a2d9a1cd3020d"),],},{"_id":ObjectId("61c6478304e98ed63e8ee7cb"),"email":"USER888@gmail.com","username":"USER999","tracks":[ObjectId("61c65353565a2d9a1cd3020d"),],}]}

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"}},{"$lookup":{"from":"users",let:{t:"$tracks.creatorId"},pipeline:[{$match:{$expr:{"$in":["$_id","$$t"]}}}],"as":"createdBy"}},])