fix case where AutoString is being converted

Author: svlandeg

sqlmodel/main.py

@@ -39,6 +39,7 @@
     Numeric,
     inspect,
 )
+from sqlalchemy import types as sa_types
 from sqlalchemy import Enum as sa_Enum
 from sqlalchemy.orm import (
     Mapped,
@@ -656,6 +657,10 @@ def get_sqlalchemy_type(field: Any) -> Any:
     type_ = get_sa_type_from_field(field)
     metadata = get_field_metadata(field)
 
+    # If it's already an SQLAlchemy type (eg. AutoString), use it directly
+    if isinstance(type_, type) and issubclass(type_, sa_types.TypeEngine):
+        return type_
+
     # Checks for `Literal` type annotation
     if type_ is Literal:
         return AutoString