CAST in sql only when strincly necessary, still, can be better

VERSION

@@ -1 +1 @@
-Version 2.5.1-stable+timestamp.2013.06.09.16.56.30
+Version 2.5.1-stable+timestamp.2013.06.09.17.00.11

gluon/dal.py

@@ -1357,7 +1357,8 @@ class BaseAdapter(ConnectionPool):
     def expand(self, expression, field_type=None):
         if isinstance(expression, Field):
             out = '%s.%s' % (expression.tablename, expression.name)
-            if field_type == 'string':
+            if field_type == 'string' and not expression.type in (
+                'string','text','json','password'):
                 out = 'CAST(%s AS %s)' % (out, self.types['text'])
             return out
         elif isinstance(expression, (Expression, Query)):