two-queries.rb

posts      = Post.all
author_ids = posts.pluck(:author_id)
authors    = User.where(:_id.in => author_ids)
# Somehow pass the author objects to the post serializer and
# map them to the correct post objects. Can't imagine what 
# exactly that would look like, but probably not pretty.
render json: posts, pass_some_parameter_maybe: authors