[IMP] Add action for choosing a variant as the price reference

- This can be very handy for the website if we want to use not the cheapest variant as the default
  displayed price. It also reorders the attribute values such that when opening a product on the
  website, it immediately shows the reference variant.

product_variant_sale_price/__manifest__.py

@@ -12,6 +12,6 @@
     "license": "AGPL-3",
     "installable": True,
     "depends": ["account", "sale"],
-    "data": ["views/product_views.xml"],
+    "data": ["data/server_actions.xml", "views/product_views.xml"],
     "post_init_hook": "set_sale_price_on_variant",
 }

product_variant_sale_price/data/server_actions.xml

@@ -0,0 +1,17 @@
+<odoo>
+    <record model="ir.actions.server" id="set_price_reference">
+        <field name="name">Set price reference</field>
+        <field name="model_id" ref="model_product_product" />
+        <field name="state">code</field>
+        <field name="binding_model_id" ref="model_product_product" />
+        <field name="binding_type">action</field>
+        <field name="code">
+if len(records) > len(records.mapped("product_tmpl_id")):
+    raise UserError("You can only set one variant as the price reference of a product.")
+for r in records.with_context(skip_update_fix_price=True):
+    r.product_tmpl_id.write({"list_price": r.fix_price})
+    new_seq = min(r.product_tmpl_id.attribute_line_ids.mapped("value_ids.sequence") or [r.fix_price])
+    r.mapped("product_template_variant_value_ids.product_attribute_value_id").write({"sequence": new_seq - 1})
+        </field>
+    </record>
+</odoo>

product_variant_sale_price/models/product_product.py

@@ -27,11 +27,24 @@ def write(self, vals):
             template._update_fix_price(vals)
         return res
 
-    def _get_combination_info(self, combination=False, product_id=False, add_qty=1, pricelist=False,
-                              parent_combination=False, only_template=False):
-        res = super()._get_combination_info(combination, product_id, add_qty, pricelist,
-                                            parent_combination, only_template)
-        res['price_extra'] = 0.0
+    def _get_combination_info(
+        self,
+        combination=False,
+        product_id=False,
+        add_qty=1,
+        pricelist=False,
+        parent_combination=False,
+        only_template=False,
+    ):
+        res = super()._get_combination_info(
+            combination,
+            product_id,
+            add_qty,
+            pricelist,
+            parent_combination,
+            only_template,
+        )
+        res["price_extra"] = 0.0
         return res
 
 
@@ -88,7 +101,8 @@ def _inverse_product_lst_price(self):
             product.write(vals)
 
     def _compute_product_price_extra(self):
-        """the sale.order.line module calculates the price_unit by adding the value of price_extra and this can
-        generate inconsistencies if the field has old data stored."""
+        """the sale.order.line module calculates the price_unit by adding
+        the value of price_extra and this can generate inconsistencies
+        if the field has old data stored."""
         for product in self:
             product.price_extra = 0.0