diff --git a/optimade/server/entry_collections/entry_collections.py b/optimade/server/entry_collections/entry_collections.py
index a963ccdd48..fb4d101605 100644
--- a/optimade/server/entry_collections/entry_collections.py
+++ b/optimade/server/entry_collections/entry_collections.py
@@ -319,9 +319,6 @@ def handle_query_params(
             for f in self.all_fields
         }
 
-        if "_id" not in cursor_kwargs["projection"]:
-            cursor_kwargs["projection"]["_id"] = False
-
         if getattr(params, "response_fields", False):
             response_fields = set(params.response_fields.split(","))
             response_fields |= self.resource_mapper.get_required_fields()
diff --git a/optimade/server/entry_collections/mongo.py b/optimade/server/entry_collections/mongo.py
index 2da6120f94..97587534f1 100644
--- a/optimade/server/entry_collections/mongo.py
+++ b/optimade/server/entry_collections/mongo.py
@@ -113,11 +113,16 @@ def _run_db_query(
 
         """
 
-        results = []
-        for doc in self.collection.find(**criteria):
-            if criteria.get("projection", {}).get("_id"):
-                doc["_id"] = str(doc["_id"])
-            results.append(doc)
+        # Handle MongoDB ObjectIDs:
+        # - If they were not requested, then explicitly remove them
+        # - If they were requested, then cast them to strings in the response
+        if "_id" not in criteria["projection"]:
+            criteria["projection"]["_id"] = False
+
+        if criteria.get("projection", {}).get("_id"):
+            criteria["projection"]["_id"] = {"$toString": "$_id"}
+
+        results = list(self.collection.find(**criteria))
 
         nresults_now = len(results)
         if not single_entry: