salesman field readonly when not draft

sale.py

@@ -5,6 +5,7 @@
 from trytond.model import fields
 from trytond.pool import PoolMeta
 from trytond.transaction import Transaction
+from trytond.pyson import Eval
 
 
 __all__ = [
@@ -16,7 +17,11 @@ __metaclass__ = PoolMeta
 class Sale():
     __name__ = 'sale.sale'
 
-    salesman = fields.Many2One('res.user', 'Salesman')
+    salesman = fields.Many2One('res.user', 'Salesman',
+        states={
+            'readonly': Eval('state') != 'draft',
+            },
+        depends=['state'])
 
     @staticmethod
     def default_salesman():