[FIX] sale: fix rounded discount

Steps to reproduce:
- Install and launch sales
- In settings: activate price lists and discount; price list based on advanced rules
- Create a new price list; based on percentage (54%)*; applicable on all products; show discount
- Make sure the Public Price List has show discount activated
- Create a new quotation
- Select the Test pricelist
- Select a product (price 0.03 eur/$/other)*
- Change the price list to Public price list
- update the prices
- change the price list to Test price list
- update the prices
-> The Discount shown is not the one defined in the price list (66.67%)*
*(values for my example)

Solution:
Use the real price and not the a posteriori-rounded price for the computation of the discount  in the `update_prices` method.

opw-2677884

closes odoo/odoo#81395

X-original-commit: 986dbc20952caae9d5a1b4b7ee34cfb994dea077
Signed-off-by: yosa-odoo <yosa@odoo.com>

addons/sale/models/sale_order.py

@@ -534,7 +534,8 @@ class SaleOrder(models.Model):
             price_unit = self.env['account.tax']._fix_tax_included_price_company(
                 line._get_display_price(product), line.product_id.taxes_id, line.tax_id, line.company_id)
             if self.pricelist_id.discount_policy == 'without_discount' and price_unit:
-                discount = max(0, (price_unit - product.price) * 100 / price_unit)
+                price_discount_unrounded = self.pricelist_id.get_product_price(product, line.product_uom_qty, self.partner_id, self.date_order, line.product_uom.id)
+                discount = max(0, (price_unit - price_discount_unrounded) * 100 / price_unit)
             else:
                 discount = 0
             lines_to_update.append((1, line.id, {'price_unit': price_unit, 'discount': discount}))

addons/sale/tests/test_sale_pricelist.py

@@ -350,3 +350,64 @@ class TestSaleOrder(TestSaleCommon):
         self.assertEqual(order_line.price_unit, 180, "First pricelist rule not applied")
         order_line.product_uom = new_uom
         self.assertEqual(order_line.price_unit, 1800, "First pricelist rule not applied")
+
+    def test_sale_change_of_pricelists_excluded_value_discount(self):
+        """ Test SO with the pricelist 'discount displayed' and check displayed percentage value after multiple changes of pricelist """
+
+        # Create a pricelist without discount policy: percentage on all products
+        pricelist_discount_excl_global = self.env['product.pricelist'].create({
+            'name': 'Pricelist C',
+            'discount_policy': 'without_discount',
+            'company_id': self.env.company.id,
+            'item_ids': [(0, 0, {
+                'applied_on': '3_global',
+                'compute_price': 'percentage',
+                'percent_price': 54,
+            })],
+        })
+
+        # Create a product with a very low price
+        amazing_product = self.env['product.product'].create({
+            'name': 'Amazing Product',
+            'lst_price': 0.03,
+        })
+
+        # create a simple Sale Order with a unique line
+        sale_order = self.env['sale.order'].create({
+            'partner_id': self.partner_a.id,
+            'partner_invoice_id': self.partner_a.id,
+            'partner_shipping_id': self.partner_a.id,
+            'pricelist_id': self.company_data['default_pricelist'].id,
+            'order_line': [(0, 0, {
+                'name': amazing_product.name,
+                'product_id': amazing_product.id,
+                'product_uom_qty': 1,
+                'product_uom': amazing_product.uom_id.id,
+                'price_unit': 0.03,
+                'tax_id': False,
+            })],
+        })
+
+        # Change the pricelist
+        sale_order.write({'pricelist_id': pricelist_discount_excl_global.id})
+        # Update Prices
+        sale_order.update_prices()
+
+        # Check that the discount displayed is the correct one
+        self.assertEqual(
+            sale_order.order_line.discount, 54,
+            "Wrong discount computed for specified product & pricelist"
+        )
+        # Additional to check for overall consistency
+        self.assertEqual(
+            sale_order.order_line.price_unit, 0.03,
+            "Wrong unit price computed for specified product & pricelist"
+        )
+        self.assertEqual(
+            sale_order.order_line.price_subtotal, 0.01,
+            "Wrong subtotal price computed for specified product & pricelist"
+        )
+        self.assertFalse(
+            sale_order.order_line.tax_id,
+            "Wrong tax applied for specified product & pricelist"
+        )