Replace domain on cost_price_method by an on_change_type

Function fields like cost_price_method can not have a domain if they do not
have a searcher.

issue2356
review36641002

product.py

@@ -7,7 +7,7 @@ from decimal import Decimal
 from sql import Null, Column
 
 from trytond.model import ModelView, ModelSQL, Model, fields
-from trytond.pyson import Eval, If
+from trytond.pyson import Eval
 from trytond.transaction import Transaction
 from trytond.pool import Pool
 from trytond import backend
@@ -60,11 +60,7 @@ class Template(ModelSQL, ModelView, CompanyMultiValueMixin):
             "Cost Price", digits=price_digits), 'get_cost_price')
     cost_price_method = fields.MultiValue(fields.Selection(
             COST_PRICE_METHODS, "Cost Price Method", required=True,
-            domain=[If((Eval('type') == 'service'),
-                    ('cost_price_method', '=', 'fixed'),
-                    (),
-                )
-            ], states=STATES, depends=DEPENDS + ['type']))
+            states=STATES, depends=DEPENDS))
     cost_price_methods = fields.One2Many(
         'product.cost_price_method', 'template', "Cost Price Methods")
     default_uom = fields.Many2One('product.uom', 'Default UOM', required=True,
@@ -146,6 +142,11 @@ class Template(ModelSQL, ModelView, CompanyMultiValueMixin):
             return []
         return [{}]
 
+    @fields.depends('type', 'cost_price_method')
+    def on_change_type(self):
+        if self.type == 'service':
+            self.cost_price_method = 'fixed'
+
     @fields.depends('default_uom')
     def on_change_with_default_uom_category(self, name=None):
         if self.default_uom: