class Module

def thread_mattr_writer(*syms, instance_writer: true, instance_accessor: true, default: nil) # :nodoc:

:nodoc:
Current.new.user = "DHH" # => NoMethodError

end
thread_mattr_writer :user, instance_writer: false
class Current

instance_writer: false or instance_accessor: false.
To omit the instance writer method, pass

Thread.current[:attr_Current_user] # => "DHH"
Current.user = "DHH"

end
thread_mattr_writer :user
module Current

allow assignment to the attribute.
Defines a per-thread class attribute and creates a class and instance writer methods to 
def thread_mattr_writer(*syms, instance_writer: true, instance_accessor: true, default: nil) # :nodoc:
  syms.each do |sym|
    raise NameError.new("invalid attribute name: #{sym}") unless /^[_A-Za-z]\w*$/.match?(sym)
    # The following generated method concatenates `name` because we want it
    # to work with inheritance via polymorphism.
    class_eval(<<-EOS, __FILE__, __LINE__ + 1)
      def self.#{sym}=(obj)
        @__thread_mattr_#{sym} ||= "attr_\#{name}_#{sym}"
        ::ActiveSupport::IsolatedExecutionState[@__thread_mattr_#{sym}] = obj
      end
    EOS
    if instance_writer && instance_accessor
      class_eval(<<-EOS, __FILE__, __LINE__ + 1)
        def #{sym}=(obj)
          self.class.#{sym} = obj
        end
      EOS
    end
    public_send("#{sym}=", default) unless default.nil?
  end
end