[FIX] purchase: use Product Price precision in bills

When the Product Price precision is greater than the currency precision,
it can lead to incorrect data

To reproduce the error:
(Enable debug mode)
1. Settings > Technical > Database Structure > Decimal Accuracy, edit
Product Price:
    - Digits: 3
2. Create a PO
    - Add a product:
        - Quantity: 12
        - Unit Price: 0.001
3. Save, Confirm, Edit the PO:
    - Qty Received: 12
    - (Note that the total is $0.01)
4. Create a bill:
    - Add the PO to the field "Auto-Complete"

Error: The unit price is $0.000 and so does the total

The rounding of the unit price should be based on the Product Price
precision, not the currency precision.

OPW-2601867

closes odoo/odoo#74813

X-original-commit: 1123856c77cce4b69059b63c2bcbb382c7f0719e
Signed-off-by: Rémy Voet <ryv-odoo@users.noreply.github.com>
Signed-off-by: Adrien Widart <adwid@users.noreply.github.com>

addons/purchase/models/purchase.py

@@ -1260,7 +1260,7 @@ class PurchaseOrderLine(models.Model):
             'product_id': self.product_id.id,
             'product_uom_id': self.product_uom.id,
             'quantity': self.qty_to_invoice,
-            'price_unit': self.currency_id._convert(self.price_unit, aml_currency, self.company_id, date),
+            'price_unit': self.currency_id._convert(self.price_unit, aml_currency, self.company_id, date, round=False),
             'tax_ids': [(6, 0, self.taxes_id.ids)],
             'analytic_account_id': self.account_analytic_id.id,
             'analytic_tag_ids': [(6, 0, self.analytic_tag_ids.ids)],

addons/purchase/tests/test_purchase_invoice.py

@@ -275,3 +275,27 @@ class TestPurchaseToInvoice(AccountTestInvoicingCommon):
         self.assertEqual(len(amls), 2)
         self.assertEqual(amls[0].amount_currency, 1000)
         self.assertEqual(amls[1].amount_currency, 500)
+
+    def test_product_price_decimal_accuracy(self):
+        self.env['ir.model.data'].xmlid_to_object('product.decimal_price').digits = 3
+        self.env.company.currency_id.rounding = 0.01
+
+        po = self.env['purchase.order'].with_context(tracking_disable=True).create({
+            'partner_id': self.partner_a.id,
+            'order_line': [(0, 0, {
+                'name': self.product_a.name,
+                'product_id': self.product_a.id,
+                'product_qty': 12,
+                'product_uom': self.product_a.uom_id.id,
+                'price_unit': 0.001,
+                'taxes_id': False,
+            })]
+        })
+        po.button_confirm()
+        po.order_line.qty_received = 12
+
+        move_form = Form(self.env['account.move'].with_context(default_move_type='in_invoice'))
+        move_form.purchase_vendor_bill_id = self.env['purchase.bill.union'].browse(-po.id)
+        move = move_form.save()
+
+        self.assertEqual(move.amount_total, 0.01)