db={"users":[{_id:1,username:"A",password:"A",privacy_level:"public"}],"posts":[{_id:1,user_id:1,text:"Demo",timestamp:new Date()}]}

db.posts.aggregate([{$lookup:{from:"users",localField:"user_id",foreignField:"_id",as:"user"}},{$match:{"user.privacy_level":"public"}},{$project:{user:0,user_id:0}}])