qualified_const_defined?

# File activesupport/lib/active_support/dependencies.rb, line 114
  def qualified_const_defined?(path)
    raise NameError, "#{path.inspect} is not a valid constant name!" unless
      /^(::)?([A-Z]\w*)(::[A-Z]\w*)*$/ =~ path

    names = path.to_s.split('::')
    names.shift if names.first.empty?

    # We can't use defined? because it will invoke const_missing for the parent
    # of the name we are checking.
    names.inject(Object) do |mod, name|
      return false unless uninherited_const_defined?(mod, name)
      mod.const_get name
    end
    return true
  end