docs: def validate_username(self, data, value):
          """ Ensure the username is unique
  
          If the login is being created then simply check if
          the username is in the store & fail.
  
          Otherwise if the login is being updated check if the
          existing rid on a username match is the same as the
          login being updated otherwise fail.
          """
  
          store = goldman.sess.store
          existing = store.find(data['rtype'], 'username', value)
  
          if existing:
              if not data['rid'] or data['rid'] != existing.rid:
                  raise ValidationError('username is already taken')