If you prefer to keep your models namespaced then ActiveModel::Naming really makes that harder for you than it would have to be.

fix_model_name.rb

module MyApp::ModelNameFix
  def model_name
    @_model_name ||= ActiveModel::Name.new(self, nil, self.name.sub(/^#{self.parent.name}::/, ''))
  end

  def namespaced_model_name
    @_namespaced_model_name ||= ActiveModel::Name.new(self)
  end

  # Work with Draper
  def decorator_class
    prefix = self.namespaced_model_name
    decorator_name = "#{prefix}Decorator"
    decorator_name.constantize
  rescue NameError => error
    raise unless error.missing_name?(decorator_name)
    raise Draper::UninferrableDecoratorError.new(self)
  end
end

class ActiveRecord::Base
  extend MyApp::ModelNameFix
end