Refactoring of this gist: https://gist.github.com/maxrice/d744b4a5bff9ce135de9d9dd01a31664

app\controllers\application_controller.rb

class ApplicationController < ActionController::Base
  # While in such a small app, this extraction into a concern seems unnecessary.
  # I included this abstraction as an example of DRY and modular code..
  include SetRequest
end

app\controllers\concerns\set_request.rb

module SetRequest
  extend ActiveSupport::Concern

  included do
    before_action :set_user
  end

  private
    def set_user
      # ActiveSupport::CurrentAttributes is thread isolated and resets before
      # and after each request. Since it the user is only being set once during
      # each request, memoization is not needed.
      Current.user = User.find(session[:id])
    end
end

app\controllers\todo_items_controller.rb

class TodoItemsController < ApplicationController
  # This was moved to its own controller to match RESTful route principles. It
  # was also renamed from "complete_item" to "complete" to make a prettier URL.
  #
  # ex. /todo_items/:id/complete
  #
  # Routes configuration not included as instructed by the original Gist
  # description
  def complete
    @item = TodoList.find(params[:id])
    @item.complete!

    # Rails 5 introduces the improved "redirect_back" to replace the old
    # "redirect_to :back", which was deprecated. This counters the
    # ActionController::RedirectBackError exception when HTTP_REFERER is not
    # present
    redirect_back fallback_location: @item.todo_list
  end
end

app\controllers\todo_lists_controller.rb

class TodoListsController < ApplicationController
  before_action :set_list, only: [:show, :complete_item]

  def index
    # "TodoList.where(user_id: User.current_user.id)" was not as readable as the
    # following code
    @lists = Current.user.todo_lists.order("#{sort_by} #{sort_order}")
  end

  def show
    @list = TodoList.find(params[:id])
  end

  def new
    @list = TodoList.new
  end

  def create
    @list = TodoList.new(todo_list_params)

    if @list.save!
      redirect_to todo_lists_url
    else
      # Provide the user with helpful error messages
      flash[:notice] = "Error: #{@list.errors.full_messages.to_sentence}"
      redirect_to new_todo_list_url
    end
  end

  private
    # Never trust parameters from the scary internet. I figured we add some
    # strong parameters
    def todo_list_params
      # I made some guesses on the structure of TodoList based on the model and
      # views
      params.require(:todo_list).permit(:title)
    end

    # "sort_by" and "sort_order" were refactored using the Extract Method
    def sort_by
      params[:sort] || 'created_at'
    end

    def sort_order
      (params[:asc] == 'false') ? 'ASC' : 'DESC'
    end
end

app\models\current.rb

class Current < ActiveSupport::CurrentAttributes
  attribute :user
end

app\models\todo_item.rb

class TodoItem < ApplicationRecord
  belongs_to :todo_list

  validates :title, presence: true

  scope :incomplete, -> { where(done: false) }

  # Completes to do item, saving to database.
  def complete!
    update!(done: true)
  end
end

app\models\todo_list.rb

class TodoList < ApplicationRecord
  has_many :todo_items
  belongs_to :user
  
  before_validation :set_user, on: :create
  
  def formatted_title
    title + (todo_items.empty? ' (Complete)' : '')
  end
  
  # Assign this list to the currently signed in user when it's created
  # (Personal preference) I find that keeping the method and variable names
  # complete, rather than abbreviated makes for more readable code
  def set_user
    self.user ||= Current.user
  end
end

app\models\user.rb

class User < ApplicationRecord
  has_many :todo_lists
end

app\views\todo_items\_todo_item.html.erb

<li>
  <%= item.title %> - <%= link_to 'Done!', complete_todo_item_path(item), method: :put %>
</li>

app\views\todo_lists\_todo_list.html.erb

<h2><%= list.formatted_title %></h2>

<ul>
  <%# extract todo_item list out into a partial %>
  <%= render list.todo_items.incomplete %>
</ul>

app\views\todo_lists\index.html.erb

<%# Display the current user's lists along with any uncompleted items %>

<%# extract todo_lists out into a partial %>
<%= render @lists %>

app\views\todo_lists\show.html.erb

<%# extract todo_list out into a partial %>
<%= render @list %>