Add commercial_partner_id fields into contract module

Remove string attribute

contract/models/contract.py

@@ -84,6 +84,14 @@ class ContractContract(models.Model):
         required=True
     )
 
+    commercial_partner_id = fields.Many2one(
+        'res.partner',
+        related='partner_id.commercial_partner_id',
+        store=True,
+        string='Commercial Entity',
+        index=True
+    )
+
     @api.multi
     def _inverse_partner_id(self):
         for rec in self: