diff --git a/src/snek/system/mapper.py b/src/snek/system/mapper.py
index 6341a29..5e4b79e 100644
--- a/src/snek/system/mapper.py
+++ b/src/snek/system/mapper.py
@@ -12,7 +12,7 @@ class BaseMapper:
 
     def __init__(self, app):
         self.app = app
-    
+        self.semaphore = asyncio.Semaphore(1)    
         self.default_limit = self.__class__.default_limit
 
     @property
@@ -24,8 +24,9 @@ class BaseMapper:
         return asyncio.get_event_loop()
 
     async def run_in_executor(self, func, *args, **kwargs):
-        return func(*args, **kwargs)
-        #return await self.loop.run_in_executor(None, lambda: func(*args, **kwargs))
+        async with self.semaphore:
+        #return func(*args, **kwargs)
+            return await self.loop.run_in_executor(None, lambda: func(*args, **kwargs))
 
     async def new(self):
         return self.model_class(mapper=self, app=self.app)