[IMP] product: ensure variant price is managed on the template

* variant prices should be read only on the variant
* added an help to explain how it works
* because variant prices are managed on the template via the Variant
  Prices button

addons/product/models/product.py

@@ -116,7 +116,8 @@ class ProductProduct(models.Model):
         help="This is the sum of the extra price of all attributes")
     lst_price = fields.Float(
         'Sale Price', compute='_compute_product_lst_price',
-        digits=dp.get_precision('Product Price'), inverse='_set_product_price')
+        digits=dp.get_precision('Product Price'), inverse='_set_product_price',
+        help="The sale price is managed from the product template. Click on the 'Variant Prices' button to set the extra attribute prices.")
 
     default_code = fields.Char('Internal Reference', index=True)
     code = fields.Char('Internal Reference', compute='_compute_product_code')

addons/product/views/product_views.xml

@@ -224,7 +224,8 @@
                         </group>
                         <group>
                             <group name="pricing" string="Pricing">
-                                <field name="lst_price" widget='monetary' options="{'currency_field': 'currency_id'}"/>
+                                <field name="product_variant_count" invisible="1"/>
+                                <field name="lst_price" widget='monetary' options="{'currency_field': 'currency_id'}" attrs="{'readonly': [('product_variant_count', '&gt;', 1)]}"/>
                                 <field name="standard_price" widget='monetary' options="{'currency_field': 'currency_id'}"/>
                                 <field name="currency_id" invisible='1'/>
                             </group>
@@ -310,6 +311,7 @@
                 </field>
                 <field name="list_price" position="attributes">
                    <attribute name="name">lst_price</attribute>
+                   <attribute name="attrs">{'readonly': [('product_variant_count', '&gt;', 1)]}</attribute>
                 </field>
                 <field name="name" position="after">
                     <field name="product_tmpl_id" class="oe_inline" readonly="1" invisible="1" attrs="{'required': [('id', '!=', False)]}"/>