limit the most recent documents to fetch from MongoDB database. (#25435)

limit the most recent documents to fetch from MongoDB database.

Thank you for contributing to LangChain!

- [ ] **limit the most recent documents to fetch from MongoDB
database.**: "langchain_mongodb: limit the most recent documents to
fetch from MongoDB database."


- [ ] **PR message**: ***Delete this entire checklist*** and replace
with
- **Description:** Added a doc_limit parameter which enables the limit
for the documents to fetch from MongoDB database
    - **Issue:** 
    - **Dependencies:** None

---------

Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/partners/mongodb/langchain_mongodb/chat_message_histories.py

@@ -68,6 +68,7 @@ class MongoDBChatMessageHistory(BaseChatMessageHistory):
         session_id_key: str = DEFAULT_SESSION_ID_KEY,
         history_key: str = DEFAULT_HISTORY_KEY,
         create_index: bool = True,
+        history_size: Optional[int] = None,
         index_kwargs: Optional[Dict] = None,
     ):
         """Initialize with a MongoDBChatMessageHistory instance.
@@ -88,6 +89,8 @@ class MongoDBChatMessageHistory(BaseChatMessageHistory):
                 name of the field that stores the chat history.
             create_index: Optional[bool]
                 whether to create an index on the session id field.
+            history_size: Optional[int]
+                count of (most recent) messages to fetch from MongoDB.
             index_kwargs: Optional[Dict]
                 additional keyword arguments to pass to the index creation.
         """
@@ -97,6 +100,7 @@ class MongoDBChatMessageHistory(BaseChatMessageHistory):
         self.collection_name = collection_name
         self.session_id_key = session_id_key
         self.history_key = history_key
+        self.history_size = history_size
 
         try:
             self.client: MongoClient = MongoClient(connection_string)
@@ -114,7 +118,15 @@ class MongoDBChatMessageHistory(BaseChatMessageHistory):
     def messages(self) -> List[BaseMessage]:  # type: ignore
         """Retrieve the messages from MongoDB"""
         try:
-            cursor = self.collection.find({self.session_id_key: self.session_id})
+            if self.history_size is None:
+                cursor = self.collection.find({self.session_id_key: self.session_id})
+            else:
+                skip_count = max(
+                    0, self.collection.count_documents({}) - self.history_size
+                )
+                cursor = self.collection.find(
+                    {self.session_id_key: self.session_id}, skip=skip_count
+                )
         except errors.OperationFailure as error:
             logger.error(error)
 

libs/partners/mongodb/tests/unit_tests/test_chat_message_histories.py

@@ -16,6 +16,7 @@ class PatchedMongoDBChatMessageHistory(MongoDBChatMessageHistory):
         self.collection = MockCollection()
         self.session_id_key = "SessionId"
         self.history_key = "History"
+        self.history_size = None
 
 
 def test_memory_with_message_store() -> None: