app\controllers\todo_list_controller.rb

class TodoListsController < ApplicationController

  before_action :set_current_user
  before_action :set_todo_list, only: %i[show complete_item] 
  
  ORDER_OPTIONS = ['created_at', 'updated_at']

  def index
    sort_order = (params[:asc] == 'false') ? 'ASC' : 'DESC'
    @lists = @user.todo_lists.order("#{sort_by_field} #{sort_order}")
  end
  
  def show
  end
  
  # Mark an item as done and return user back from whence they came!
  def complete_item
    @list.complete!
    redirect_to :back
  end

  def new
    @list = TodoList.new
  end

  def create
    @list = @user.todo_lists.build(todo_list_params)

    if @list.save!
      redirect_to todo_lists_url
    else
      flash[:notice] = "Error saving list"
      redirect_to new_todo_list_url
    end
  end
  
  private 

  def set_current_user
    @user ||= User.find(session[:user_id])
  end

  def set_todo_list
    @list = @user.todo_lists.find(params[:id])
  end

   def sort_by_field
     sort_field = params[:sort] if params[:sort] && ORDER_OPTIONS.include?(params[:sort])
     sort_field ||= 'created_at'
     sort_field
   end
  
  def todo_list_params
    params.require(:todo_list).permit(
      :title,
      :done
    )
  end
end

app\helpers\todo_lists_helper.rb

module TodoListsHelper
  
  def formatted_title(list)
    list.title + (list.todo_items.empty? ' (Complete)' : '')
  end
    
end

app\models\todo_item.rb

class TodoItem < ApplicationRecord

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

  belongs_to :todo_list

  validates :title, presence: true

  # 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
  
end

app\models\user.rb

class User < ApplicationRecord
  has_many :todo_lists
  
end

app\views\todo_lists\list.erb

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

<% @lists.each do |list| %>
  <h2><%= formatted_title(list) %></h2>
  <ul>
    <% list.todo_items.uncompleted.each do |item| %>
      <li><% item.title %> - <%= link_to "Done!", complete_item_todo_lists(item), method: :put %></li>
    <% end %>
  </ul>
<% end %>

app\views\todo_lists\show.erb

<h2><%= formatted_title(@list) %></h2>
<ul>
  <% @list.todo_items.uncompleted.each do |item| %>
    <li><% item.title %> - <%= link_to "Done!", complete_item_todo_lists(item), method: :put %></li>
  <% end %>
</ul>