'Check if ``activation_key`` is still valid.
Raises a ``self.model.DoesNotExist`` exception if key is not present or
``activation_key`` is not a valid string
:param activation_key:
String containing the secret SHA1 for a valid activation.
:return:
True if the ket has expired, False if still valid.'

def check_expired_activation(self, activation_key):

    if SHA1_RE.search(activation_key):
        userena = self.get(activation_key=activation_key)
        return userena.activation_key_expired()
    raise self.model.DoesNotExist
