code: def exists(self, value=None):
          
          try:
              if not value:
                  value = self.get()
          except (AttributeError, DoesNotExist):
              # If the instance is deleted, the _pk attribute doesn't exist
              # anymore. So we catch the AttributeError to return False (this pk
              # field doesn't exist anymore) in this specific case
              return False
          else:
              return self.connection.sismember(self.collection_key, value)
docs: Return True if the given pk value exists for the given class.
          If no value is given, we use the value of the current field, which
          is the value of the "_pk" attribute of its instance.