add constrain a serial unique

equipment.py

@@ -1,6 +1,6 @@
 from collections import defaultdict
 from trytond.pool import Pool
-from trytond.model import ModelSQL, ModelView, fields
+from trytond.model import ModelSQL, ModelView, Unique, fields
 from trytond.exceptions import UserError
 
 class OpticalEquipment(ModelSQL, ModelView):
@@ -36,3 +36,12 @@ class OpticalEquipment(ModelSQL, ModelView):
     @staticmethod
     def get_origin():
         return None
+
+    @classmethod
+    def __setup__(cls):
+        super(OpticalEquipment, cls).__setup__()
+        t = cls.__table__()
+        cls._sql_constraints = [
+            ('serial_unique', Unique(t, t.serial),
+             'optical_equipment.msg_serial_unique')
+            ]

message.xml

@@ -8,5 +8,8 @@ this repository contains the full copyright notices and license terms. -->
 	<record model="ir.message" id="msg_invalid_number_purchases">
             <field name="text">Please, select only one purchase.</field>
         </record>
+	<record model="ir.message" id="msg_serial_unique">
+            <field name="text">The serial number of equipment, should be unique.</field>
+        </record>
     </data>
 </tryton>

purchase.py

@@ -37,7 +37,7 @@ class Purchase(metaclass=PoolMeta):
                         company=line.company,
                         location=line.to_location,
                         equipment_type=line.product.equipment_type,
-                        propietary=line.company,
+                        propietary=line.company.party,
                         propietary_address=line.address_equipment,
                         product=line.product,
                         model_category=line.product.model_category,