def put(self, request):
''' Create a new custom URL. If the url is already in use, error.'''
url_key = ndb.Key('CustomURL', request.slug)
custom_url = url_key.get()
if custom_url != None:
# Url is already in use.
raise remote.ApplicationError('Custom URL already exists.')
target = ndb.Key(urlsafe=self.decrypt(request.target))
target_obj = target.get()
if not target_obj:
# Target object not found.
raise remote.ApplicationError('put url requires a valid target')
custom_url = CustomURL(key=url_key, slug=request.slug, target=target)
custom_url.put()
target_obj.customurl = url_key
target_obj.put()
return custom_url.to_message()
