Write the dirty attributes to the data store.

def _WriteAttributes(self):
    """Write the dirty attributes to the data store."""
    # If the object is not opened for writing we do not need to flush it to the
    # data_store.
    if "w" not in self.mode:
      return

    if self.urn is None:
      raise ValueError("Storing of anonymous AFF4 objects not supported.")

    to_set = {}
    for attribute_name, value_array in iteritems(self.new_attributes):
      to_set_list = to_set.setdefault(attribute_name, [])
      for value in value_array:
        to_set_list.append((value.SerializeToDataStore(), value.age))

    if self._dirty:
      # We determine this object has a new version only if any of the versioned
      # attributes have changed. Non-versioned attributes do not represent a new
      # object version. The type of an object is versioned and represents a
      # version point in the life of the object.
      if self._new_version:
        to_set[self.Schema.TYPE] = [
            (utils.SmartUnicode(self.__class__.__name__),
             rdfvalue.RDFDatetime.Now())
        ]

      # We only update indexes if the schema does not forbid it and we are not
      # sure that the object already exists.
      add_child_index = self.Schema.ADD_CHILD_INDEX
      if self.object_exists:
        add_child_index = False

      # Write the attributes to the Factory cache.
      FACTORY.SetAttributes(
          self.urn,
          to_set,
          self._to_delete,
          add_child_index=add_child_index,
          mutation_pool=self.mutation_pool)