Nesting a resource under its owner's username

application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception
  
private
  # use default_url_options ?
  def item_path(item)
    super(item.user, item)
  end
  helper_method :item_path
end

index.html.erb

<ul>
  <% @items.each do |item| %>
    <li><%= link_to item.name, item_path(item) %>
  <% end %>
</ul>

item.rb

class Item < ActiveRecord::Base
  belongs_to :user
end

items_controller.rb

class ItemsController < ApplicationController
  before_action :set_user

  def index
    @items = @user.items
  end
 
  def create
    @item = @user.items.new(item_params)
    if @item.save
      # The `redirect_to @item` shortcut doesn't work
      redirect_to item_path(@item)
    else
      render :new
    end
  end

private

  def set_user
    @user = User.find_by(username: params[:username])
  end
end

routes.rb

MyApp::Application.routes.draw do
  resources :items, path: ':username'
end

user.rb

class User < ActiveRecord::Base
  has_many :items

  before_validation :downcase_username, if: :username_changed?

  validates :username, presence: true,
    uniqueness: true,
    format: { with: /\A[a-z0-9_]+\z/, allow_blank: true }

  def to_param
    username
  end

private

  def downcase_username
    self.username = username.dowcase
  end
end