db={"post":[{"_id":"somepost1","author":"firstuser","title":"First post"},{"_id":"somepost2","author":"firstuser","title":"Second post"},{"_id":"somepost3","author":"firstuser","title":"Third post"}],"user":[{"_id":"firstuser","nickname":"John","posts":{"voted":[]}},{"_id":"seconduser","nickname":"Bob","posts":{"voted":[{"_id":"somepost1","vote":"1"},{"_id":"somepost3","vote":"-1"}]}}]}

db.post.aggregate([{$match:{author:"firstuser"}},{$lookup:{from:"user",localField:"author",foreignField:"_id",as:"author"}},{$addFields:{author:{$arrayElemAt:["$author",0]}}},{$lookup:{from:"user",localField:"_id",foreignField:"posts.voted._id",as:"Results"}},{$unwind:"$Results"},{$unwind:"$Results.posts.voted"},{$match:{"Results.posts.voted._id":"ID OF CURRENT POST"}},{$project:{_id:1,author:{_id:1,nickname:1},title:1,myvote:"$Results.posts.voted.vote"}}])