From 5431badfd4ebdb56ebf12b6410a14417043125be Mon Sep 17 00:00:00 2001
From: Kartik <kartik.labhshetwar@mem0.ai>
Date: Tue, 24 Mar 2026 10:58:21 +0530
Subject: [PATCH] fix: preserve custom metadata when updating memory (#4495)

---
 mem0/vector_stores/mongodb.py       |  3 +-
 tests/vector_stores/test_mongodb.py | 46 ++++++++++++++++++++++++++++-
 2 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/mem0/vector_stores/mongodb.py b/mem0/vector_stores/mongodb.py
index 7d1e82bb6..b9cf0aa6b 100644
--- a/mem0/vector_stores/mongodb.py
+++ b/mem0/vector_stores/mongodb.py
@@ -197,7 +197,8 @@ class MongoDB(VectorStoreBase):
         if vector is not None:
             update_fields["embedding"] = vector
         if payload is not None:
-            update_fields["payload"] = payload
+            for key, value in payload.items():
+                update_fields[f"payload.{key}"] = value
 
         if update_fields:
             try:
diff --git a/tests/vector_stores/test_mongodb.py b/tests/vector_stores/test_mongodb.py
index d75a05846..33a8a7ad0 100644
--- a/tests/vector_stores/test_mongodb.py
+++ b/tests/vector_stores/test_mongodb.py
@@ -203,6 +203,10 @@ def test_delete(mongo_vector_fixture):
 
 
 def test_update(mongo_vector_fixture):
+    """
+    Test that update() uses dot notation for payload fields instead of replacing
+    the entire payload document.
+    """
     mongo_vector, mock_collection, _ = mongo_vector_fixture
     vector_id = "id1"
     updated_vector = [0.3] * 1536
@@ -212,11 +216,51 @@ def test_update(mongo_vector_fixture):
 
     mongo_vector.update(vector_id=vector_id, vector=updated_vector, payload=updated_payload)
 
+    # Should use dot notation (payload.name) instead of full replacement (payload)
     mock_collection.update_one.assert_called_once_with(
-        {"_id": vector_id}, {"$set": {"embedding": updated_vector, "payload": updated_payload}}
+        {"_id": vector_id}, {"$set": {"embedding": updated_vector, "payload.name": "updated_vector"}}
     )
 
 
+def test_update_payload_only_uses_dot_notation(mongo_vector_fixture):
+    """
+    Test that updating only the payload uses dot notation for each field,
+    preserving existing metadata fields not included in the update.
+    """
+    mongo_vector, mock_collection, _ = mongo_vector_fixture
+
+    mock_collection.update_one.return_value = MagicMock(matched_count=1)
+
+    mongo_vector.update(
+        vector_id="id1",
+        payload={"data": "updated text", "hash": "def456", "updated_at": "2025-06-01"},
+    )
+
+    set_arg = mock_collection.update_one.call_args[0][1]["$set"]
+
+    # Only the specified fields should be in $set, using dot notation
+    assert set_arg == {
+        "payload.data": "updated text",
+        "payload.hash": "def456",
+        "payload.updated_at": "2025-06-01",
+    }
+    # "payload" key itself should NOT appear (that would replace the whole document)
+    assert "payload" not in set_arg
+
+
+def test_update_vector_only_does_not_touch_payload(mongo_vector_fixture):
+    """Test that updating only the vector does not touch any payload fields."""
+    mongo_vector, mock_collection, _ = mongo_vector_fixture
+
+    mock_collection.update_one.return_value = MagicMock(matched_count=1)
+
+    mongo_vector.update(vector_id="id1", vector=[0.7, 0.8, 0.9])
+
+    set_arg = mock_collection.update_one.call_args[0][1]["$set"]
+    assert set_arg == {"embedding": [0.7, 0.8, 0.9]}
+    assert not any(k.startswith("payload.") for k in set_arg)
+
+
 def test_get(mongo_vector_fixture):
     mongo_vector, mock_collection, _ = mongo_vector_fixture
     vector_id = "id1"