diff --git a/mongoalchemy/query_expression.py b/mongoalchemy/query_expression.py
index e1e2537..7cad389 100644
--- a/mongoalchemy/query_expression.py
+++ b/mongoalchemy/query_expression.py
@@ -108,7 +108,7 @@ def in_(self, *values):
             in ``values``.  Produces a MongoDB ``$in`` expression.
         '''
         return QueryExpression({
-            str(self) : { '$in' : [self.get_type().wrap_value(value) for value in values] }
+            self : { '$in' : [self.get_type().wrap_value(value) for value in values] }
         })
     
     def nin(self, *values):
@@ -116,7 +116,7 @@ def nin(self, *values):
             in ``values``.  Produces a MongoDB ``$nin`` expression.
         '''
         return QueryExpression({
-            str(self) : { '$nin' : [self.get_type().wrap_value(value) for value in values] }
+            self : { '$nin' : [self.get_type().wrap_value(value) for value in values] }
         })
     
     def __str__(self):
diff --git a/test/test_query.py b/test/test_query.py
index 04f91fb..a5b7147 100644
--- a/test/test_query.py
+++ b/test/test_query.py
@@ -298,4 +298,13 @@ def qr_test_clone():
         it2.next()
     except StopIteration:
         pass
-    
\ No newline at end of file
+
+def test_resolve_fields():
+    class Resolver(Document):
+        i = IntField(db_field='j')
+        k = IntField()
+    s = get_session()
+    s.clear_collection(Resolver)
+    
+    q = s.query(Resolver).filter(Resolver.i.in_(6))
+