Given the following partially shown implementation of a Rails 5 app to manage simple todo lists for users, what improvements or changes would you make? Feel free to change or remove existing files/classes, or add new ones in your answer. Assume that any files/classes not shown, e.g.TodoItem, routes.rb, etc, exist with reasonable implementations.

app\controllers\todo_list_controller.rb

class TodoListsController < ApplicationController
  before_action :set_current_user
  
  def index
    sort_by = params[:sort] || 'created_at'
    sort_order = (params[:asc] == 'false') ? 'ASC' : 'DESC'
    @lists = TodoList.where(user_id: User.current_user.id).order("#{sort_by} #{sort_order}")
  end
  
  def show
    @list = TodoList.find(params[:id])
  end
  
  # Mark an item as done and return user back from whence they came!
  def complete_item
    TodoItem.find(params[:id]).complete!
    redirect_to :back
  end

  def new
    @list = TodoList.new
  end

  def create
    @list = TodoList.new(params[:todo_list])

    if @list.save!
      redirect_to todo_lists_url
    else
      flash[:notice] = "Error saving list"
      redirect_to new_todo_list_url
    end
  end
  
  def set_current_user
    User.set_current_user(session[:user_id])
  end
end

app\models\todo_item.rb

class TodoItem < ApplicationRecord
  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
  
  before_save :set_usr
  
  def formatted_title
    title + (todo_items.empty? ' (Complete)' : '')
  end
  
  # Assign this list to the currently signed in user when it's created
  def set_usr
    self.user_id = User.current_user.id
    save
  end
end

app\models\user.rb

class User < ApplicationRecord
  has_many :todo_lists
  
  def self.set_current_user(id)
    return if @_current_user.present?
    @_current_user = User.find(id)
  end
  
  # Avoid a DB query each time we need the currently logged in user
  def self.current_user
    @_current_user
  end
end

app\views\todo_lists\list.erb

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

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