Ruby: uninitialized constant Name (NameError)

uninitialized constant Name (NameError) means Ruby reached a reference to Name, a class, module or constant, and no definition was loaded under that name in the current namespace. In a plain Ruby script the fix is usually a missing require / require_relative or a typo. In Rails it usually means the file name does not match the class name, so the autoloader (Zeitwerk) never loads it; the message then shows the namespace where Ruby looked, for example uninitialized constant Admin::UsersController::User.

Example

puts Invoice.new.total     # invoice.rb was never required

main.rb:1:in '<main>': uninitialized constant Invoice (NameError)

Fixes

require_relative 'invoice'          # load the file that defines Invoice
require 'json'                      # constants from the standard library or gems need require too
puts JSON.generate({ ok: true })

module Admin
  class Report
    def user = ::User.first         # :: forces the top-level User, not Admin::User
  end
end

Rails file names

bin/rails zeitwerk:check     # lists files whose names do not match their constants