community[patch]: history size support for DynamoDBChatMessageHistory (…

…#16794)

**Description:** PR adds support for limiting number of messages
preserved in a session history for DynamoDBChatMessageHistory

---------

Co-authored-by: Bagatur <[email protected]>
Co-authored-by: Bagatur <[email protected]>

libs/community/langchain_community/chat_message_histories/dynamodb.py

@@ -43,6 +43,8 @@ class DynamoDBChatMessageHistory(BaseChatMessageHistory):
             table. DynamoDB handles deletion of expired items without consuming
             write throughput. To enable this feature on the table, follow the
             [AWS DynamoDB documentation](https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/time-to-live-ttl-how-to.html)
+        history_size: Maximum number of messages to store. If None then there is no
+            limit. If not None then only the latest `history_size` messages are stored.
     """
 
     def __init__(
@@ -56,6 +58,7 @@ def __init__(
         kms_key_id: Optional[str] = None,
         ttl: Optional[int] = None,
         ttl_key_name: str = "expireAt",
+        history_size: Optional[int] = None,
     ):
         if boto3_session:
             client = boto3_session.resource("dynamodb", endpoint_url=endpoint_url)
@@ -75,6 +78,7 @@ def __init__(
         self.key: Dict = key or {primary_key_name: session_id}
         self.ttl = ttl
         self.ttl_key_name = ttl_key_name
+        self.history_size = history_size
 
         if kms_key_id:
             try:
@@ -149,6 +153,9 @@ def add_message(self, message: BaseMessage) -> None:
         _message = message_to_dict(message)
         messages.append(_message)
 
+        if self.history_size:
+            messages = messages[-self.history_size :]
+
         try:
             if self.ttl:
                 import time