title: Golang mongodb aggregation
body: <p>I have a collection of users. User have an int64 "id", and lets say "avatar", "name" and an array of other users ids.
  What I want to achieve is to query SINGLE user, but instead of getting an array with his friends ids I want to get an array of his friends, containing their names and avatars. How to achieve it in golang? I have found some kind of what I need - "lookup" function, but I cannot understand how to use it right.</p>