community[minor]: added a feature to filter documents in Mongoloader (#18253)

"community: added a feature to filter documents in Mongoloader"
- **Description:** added a feature to filter documents in Mongoloader
    - **Feature:** the feature #18251
    - **Dependencies:** No
    - **Twitter handle:** https://twitter.com/im_Kushagra

libs/community/langchain_community/document_loaders/mongodb.py

@@ -1,6 +1,6 @@
 import asyncio
 import logging
-from typing import Dict, List, Optional
+from typing import Dict, List, Optional, Sequence
 
 from langchain_core.documents import Document
 
@@ -19,6 +19,7 @@ class MongodbLoader(BaseLoader):
         collection_name: str,
         *,
         filter_criteria: Optional[Dict] = None,
+        field_names: Optional[Sequence[str]] = None,
     ) -> None:
         try:
             from motor.motor_asyncio import AsyncIOMotorClient
@@ -38,6 +39,7 @@ class MongodbLoader(BaseLoader):
         self.client = AsyncIOMotorClient(connection_string)
         self.db_name = db_name
         self.collection_name = collection_name
+        self.field_names = field_names
         self.filter_criteria = filter_criteria or {}
 
         self.db = self.client.get_database(db_name)
@@ -61,17 +63,32 @@ class MongodbLoader(BaseLoader):
         """Load data into Document objects."""
         result = []
         total_docs = await self.collection.count_documents(self.filter_criteria)
-        async for doc in self.collection.find(self.filter_criteria):
+
+        # Construct the projection dictionary if field_names are specified
+        projection = (
+            {field: 1 for field in self.field_names} if self.field_names else None
+        )
+
+        async for doc in self.collection.find(self.filter_criteria, projection):
             metadata = {
                 "database": self.db_name,
                 "collection": self.collection_name,
             }
-            result.append(Document(page_content=str(doc), metadata=metadata))
+
+            # Extract text content from filtered fields or use the entire document
+            if self.field_names is not None:
+                fields = {name: doc[name] for name in self.field_names}
+                texts = [str(value) for value in fields.values()]
+                text = " ".join(texts)
+            else:
+                text = str(doc)
+
+            result.append(Document(page_content=text, metadata=metadata))
 
         if len(result) != total_docs:
             logger.warning(
-                f"Only partial collection of documents returned. Loaded {len(result)} "
-                f"docs, expected {total_docs}."
+                f"Only partial collection of documents returned. "
+                f"Loaded {len(result)} docs, expected {total_docs}."
             )
 
         return result