add view 'create_equipments' and domain on the button

purchase.py

@@ -18,17 +18,14 @@ class Purchase(metaclass=PoolMeta):
         super(Purchase, cls).__setup__()
         cls._buttons.update({
             'create_equipments': {
-                'invisible': Eval('equipment_avalaible')},
+                'invisible': ~Eval('equipment_avalaible')},
         })
 
     
     @classmethod
     @ModelView.button
     def create_equipments(cls, purchases):
-        if len(purchases) > 1:
-            raise InvalidNumberPurchases(
-                gettext('purchase.msg_invalid_number_purchases'))
-        else:
+        if len(purhcases) == 1:
             pool = Pool()
             Equipment = pool.get('optical_equipment.equipment')
             Line = pool.get('purchase.line')
@@ -52,7 +49,9 @@ class Purchase(metaclass=PoolMeta):
                 equipment.save()
             purchase.equipment_available = True
             cls.save(purchases)
-
+        else:
+            raise InvalidNumberPurchases(
+                gettext('purchase.msg_invalid_number_purchases'))
 
             
     """

view/purchase_form.xml

@@ -7,4 +7,7 @@
 	    <label name="equipment_available"/>
 	    <field name="equipment_available"/>
     </xpath>
+    <xpath expr="//button[@name='process']" position="after">
+        <button name="create_equipments"/>
+    </xpath>
 </data>