db={"post":[{_id:"post1",title:"Some title"}],"user":[{_id:"user1",posts:{voted:[{_id:"post1",vote:3},{_id:"post2",vote:2}]}}]}

db.post.aggregate([{$match:{_id:"post1"}},{$lookup:{from:"user",localField:"_id",foreignField:"posts.voted._id",as:"Results"}},{$unwind:"$Results"},{$unwind:"$Results.posts.voted"},{$match:{"Results.posts.voted._id":"post1"}},{$project:{"voted":"$Results.posts.voted.vote",title:1}}])