diff --git a/src/firebase_functions/db_fn.py b/src/firebase_functions/db_fn.py
index 410d5f8..aa7d33b 100644
--- a/src/firebase_functions/db_fn.py
+++ b/src/firebase_functions/db_fn.py
@@ -91,7 +91,7 @@ def _db_endpoint_handler(
         after = event_data["delta"]
         # Merge delta into data to generate an 'after' view of the data.
         if isinstance(before, dict) and isinstance(after, dict):
-            after = _util.prune_nones({**before, **after})
+            after = _util.prune_nones(_util.deep_merge(before, after))
         database_event_data = Change(
             before=before,
             after=after,
diff --git a/src/firebase_functions/private/util.py b/src/firebase_functions/private/util.py
index 9c521e9..0dd2eaa 100644
--- a/src/firebase_functions/private/util.py
+++ b/src/firebase_functions/private/util.py
@@ -77,6 +77,17 @@ def prune_nones(obj: dict) -> dict:
     return obj
 
 
+def deep_merge(dict1, dict2):
+    result = dict1.copy()
+    for key, value in dict2.items():
+        if isinstance(value, dict):
+            node = result.get(key, {})
+            result[key] = deep_merge(node, value)
+        else:
+            result[key] = value
+    return result
+
+
 def valid_on_call_request(request: _Request) -> bool:
     """Validate request"""
     if (_on_call_valid_method(request) and
diff --git a/tests/test_util.py b/tests/test_util.py
index 355d0c9..e524033 100644
--- a/tests/test_util.py
+++ b/tests/test_util.py
@@ -15,7 +15,7 @@
 Internal utils tests.
 """
 from os import environ, path
-from firebase_functions.private.util import firebase_config, microsecond_timestamp_conversion, nanoseconds_timestamp_conversion, is_precision_timestamp, normalize_path
+from firebase_functions.private.util import firebase_config, microsecond_timestamp_conversion, nanoseconds_timestamp_conversion, is_precision_timestamp, normalize_path, deep_merge
 import datetime as _dt
 
 test_bucket = "python-functions-testing.appspot.com"
@@ -121,3 +121,27 @@ def test_normalize_document_path():
     test_path2 = "test/document"
     assert normalize_path(test_path2) == "test/document", (
         "Failure, path should not be changed if it is already normalized.")
+
+
+def test_toplevel_keys():
+    dict1 = {"baz": {"answer": 42, "qux": "quux"}, "foo": "bar"}
+    dict2 = {"baz": {"answer": 33}}
+    result = deep_merge(dict1, dict2)
+    assert "foo" in result
+    assert "baz" in result
+
+
+def test_nested_merge():
+    dict1 = {"baz": {"answer": 42, "qux": "quux"}, "foo": "bar"}
+    dict2 = {"baz": {"answer": 33}}
+    result = deep_merge(dict1, dict2)
+    assert result["baz"]["answer"] == 33
+    assert result["baz"]["qux"] == "quux"
+
+
+def test_does_not_modify_originals():
+    dict1 = {"baz": {"answer": 42, "qux": "quux"}, "foo": "bar"}
+    dict2 = {"baz": {"answer": 33}}
+    deep_merge(dict1, dict2)
+    assert dict1["baz"]["answer"] == 42
+    assert dict2["baz"]["answer"] == 33