Redirect to page 404 when a Model.find(params[:id]) raises ActiveRecord::RecordNotFound

application_controller.rb

# app/controllers/application_controller.rb

class ApplicationController < ActionController::Base
  # Prevent CSRF attacks by raising an exception.
  # For APIs, you may want to use :null_session instead.
  protect_from_forgery with: :exception
  add_flash_types :success, :error

  #  See: 
  # https://stackoverflow.com/questions/2385799/how-to-redirect-to-a-404-in-rails
  def not_found
    raise ActionController::RoutingError.new('Not Found')
  rescue
    render_404
  end

  def render_404
    respond_to do |format|
      format.html { render :file => "#{Rails.root}/public/404",
                           # :layout => false,
                           :status => :not_found }
      format.xml  { head :not_found }
      format.any  { head :not_found }
    end
  end
end

templates_controller.rb

# app/controllers/templates_controller.rb

class TemplatesController < ApplicationController
  before_action :set_template, only: [:show, :edit, :update, :destroy]
  
  def index
    @template = Template.all
  end

  def show
  end

  def new
    @template = Template.new
  end

  def edit
  end
  
  private
    # Use callbacks to share common setup or constraints between actions.
    def set_template
      # very important to have `find_by_id` instead of `find`
      @template = Template.find_by_id(params[:id]) or not_found
    end
end

It works well as you shared above. But not with Friendly_id gem.
After reading a bit over FinderMethods on Rails 5. I found using exists? method instead of find_by_id works well. Since the gem convert IDs to Slugs (kind of...).

I have created a Gist which reflect such situation.

Thanks, your code has helped me...