UpdateBehavior

Always set the value during persistence. Notes: - On update, this will overwrite any existing value for the attribute.

Set the value only if the attribute does not currently exist on the item. Applies to: - Create: attribute is written when the item is inserted. - Update: attribute is written if it is missing on the existing item (i.e., absent or has not been set previously). Typical usage: fields that should be initialized once and then left unchanged (e.g., created timestamps, initial version, immutable IDs). Notes: - If the attribute already exists on the item during update, its value is preserved and not overwritten. - This behavior is analogous to using DynamoDB’s if_not_exists in update expressions for the attribute.