Preload ActiveRecord associations. (Rails 4.2.5)

preload.rb

# You can preload associations explicitly.
# This method also can apply to polymorphic association.
#
# See also https://github.com/rails/rails/blob/master/activerecord/lib/active_record/associations/preloader.rb
#
def index
  @comments = Comment.order(id: :desc).limit(10)
  
  # This prevents N+1 query for user and commentable associations
  ActiveRecord::Associations::Preloader.new.preload(@comments, [:user, :commentable])
end