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"}},{$addFields:{result:{$arrayElemAt:["$Results",0]}}},{"$project":{title:1,voted:{$map:{input:"$result.posts.voted",as:"item",in:{$cond:{if:{$eq:["$$item._id","post1"]},then:"$$item.vote",else:"$$REMOVE"}}}}}}])