Change field contact sale

equipment.py

@@ -1,5 +1,6 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
+import datetime
 from collections import defaultdict
 from trytond.pool import Pool
 from trytond.model import (

sale.py

@@ -44,6 +44,10 @@ class Sale(metaclass=PoolMeta):
         'invisible': (Eval('sale_type') != 'maintenance'),
         'readonly': True})
 
+    @classmethod
+    def __setup__(cls):
+        super(Sale, cls).__setup__()
+        cls.contact.states['required']=True
 
     @fields.depends('lines', 'sale_type', 'agended')
     def on_chage_sale_type(self):