fix: Skip auto index creation for async connections in sync context

Author: jeonghanjoo

mongoengine/document.py

@@ -245,8 +245,14 @@ def _get_collection(cls):
             # Ensure indexes on the collection unless auto_create_index was
             # set to False. Plus, there is no need to ensure indexes on slave.
             db = cls._get_db()
-            if cls._meta.get("auto_create_index", True) and db.client.is_primary:
-                cls.ensure_indexes()
+            if cls._meta.get("auto_create_index", True):
+                # Skip index creation for async connections in sync context
+                # They should use async_ensure_indexes() instead
+                from mongoengine.connection import is_async_connection
+
+                if not is_async_connection(cls._meta.get("db_alias")):
+                    if db.client.is_primary:
+                        cls.ensure_indexes()
 
         return cls._collection