Database
db={"articles":[{"_id":ObjectId("5dded7c18f30c402b0fac30a"),"votedUsers":[ObjectId("5dded61b84523642bc27f513"),ObjectId("5dded63c84523642bc27f514"),ObjectId("5dded64484523642bc27f515"),ObjectId("5dded64b84523642bc27f516"),ObjectId("5dded65284523642bc27f517"),ObjectId("5dded65a84523642bc27f518"),ObjectId("5dded66084523642bc27f519"),ObjectId("5dded66c84523642bc27f51a")],"title":"Article2","category":"Category1"},{"_id":ObjectId("5dded78f8f30c402b0fac309"),"votedUsers":[ObjectId("5dded60a84523642bc27f511"),ObjectId("5dded61384523642bc27f512"),ObjectId("5dded61b84523642bc27f513"),ObjectId("5dded63c84523642bc27f514"),ObjectId("5dded64484523642bc27f515"),ObjectId("5dded64b84523642bc27f516"),ObjectId("5dded65284523642bc27f517"),ObjectId("5dded65a84523642bc27f518")],"title":"Article1","category":"Category1"}],"users":[{"_id":ObjectId("5dded66c84523642bc27f51a"),"name":"User10"},{"_id":ObjectId("5dded66084523642bc27f519"),"name":"User9"},{"_id":ObjectId("5dded65a84523642bc27f518"),"name":"User8"},{"_id":ObjectId("5dded65284523642bc27f517"),"name":"User7"},{"_id":ObjectId("5dded64b84523642bc27f516"),"name":"User6"},{"_id":ObjectId("5dded64484523642bc27f515"),"name":"User5"},{"_id":ObjectId("5dded63c84523642bc27f514"),"name":"User4"},{"_id":ObjectId("5dded61b84523642bc27f513"),"name":"User3"},{"_id":ObjectId("5dded61384523642bc27f512"),"name":"User2"},{"_id":ObjectId("5dded60a84523642bc27f511"),"name":"User1"}]}
Query
db.articles.aggregate([{$match:{category:"Category1"}},{$lookup:{from:"users",localField:"votedUsers",foreignField:"_id",as:"users"}},{$project:{title:1,votedUsers:{$slice:["$users",3]},totalCountVoted:{$size:"$users"}}}])