# config/initializers/zz.rb
if Rails.env.development?
  # make sure we do not regress into slow startup time by preloading to much
  Rails.configuration.after_initialize do
    [
      ActiveRecord::Base.send(:descendants).map(&:name),
      ActionController::Base.descendants.map(&:name),
      (File.basename($0) != "rake" && defined?(Rake) && "rake"),
    ].compact.flatten.each { |c| raise "#{c} should not be loaded" }
  end
end

config/initializers/zz.rb:9:in `block (2 levels) in ': 
User should not be loaded (RuntimeError)