[FIX] purchase_requisition: compute taxes for alternative PO creation

Steps to reproduce:

- Create a request for quotation for a product with set vendor Taxes
- Create an alternative purchase order from that RFQ with Copy Product

Current Behavior:

The purchase order line of the PO associated with the created
alternative does not compute the taxes.

Expected behavior:

The taxes should be computed in the same way as if the PO was created
manually and then linked to the alternatives.

Cause of the issue:

When creating a PO manually, the taxes of each purchase order line
are computed when the product is set during the call of the
`onchange_product_id` onchange method:
https://github.com/odoo/odoo/blob/64c13193ae0fe23eb1b426f5bbcd8531e9a40967/addons/purchase/models/purchase.py#L1221
https://github.com/odoo/odoo/blob/7ed0a773b6e0711b127f236ce665c35202d973bc/addons/purchase/models/purchase.py#L1055-L1061
By contrast, PO created from the `action_create_alternative`
generate each `purchase.order.line` using `Command.create`. Therefore
these lines will not trigger the onchange method and their taxes
will not be computed.

Fix:

To KISS, we compute the taxes of each line just after the creation of
the record using the `_compute_tax_id` method.

opw-3750719

closes odoo/odoo#158300

X-original-commit: 15799594bda41bdb4b65387aa5f0d46482cb91c1
Signed-off-by: William Henrotin (whe) <whe@odoo.com>

addons/purchase_requisition/tests/test_purchase_requisition.py

@@ -487,3 +487,25 @@ class TestPurchaseRequisition(TestPurchaseRequisitionCommon):
         self.bo_requisition.company_id = new_company
         self.bo_requisition.action_in_progress()
         self.assertTrue(self.bo_requisition.name.startswith("REQ_"))
+
+    def test_taxes_for_alternative_po(self):
+        """
+            Check that PO lines of PO generated by alternative compute taxes
+        """
+        product = self.product_13
+        vendor = self.res_partner_1
+        po_form = Form(self.env['purchase.order'])
+        po_form.partner_id = vendor
+        with po_form.order_line.new() as line:
+            line.product_id = product
+            line.product_qty = 1
+        orig_po = po_form.save()
+        # Creates an alternative PO
+        action = orig_po.action_create_alternative()
+        alt_po_wizard_form = Form(self.env['purchase.requisition.create.alternative'].with_context(**action['context']))
+        alt_po_wizard_form.partner_id = vendor
+        alt_po_wizard_form.copy_products = True
+        alt_po_wizard = alt_po_wizard_form.save()
+        alt_po_id = alt_po_wizard.action_create_alternative()['res_id']
+        alt_po = self.env['purchase.order'].browse(alt_po_id)
+        self.assertEqual(orig_po.order_line.taxes_id, alt_po.order_line.taxes_id)

addons/purchase_requisition/wizard/purchase_requisition_create_alternative.py

@@ -60,6 +60,7 @@ class PurchaseRequisitionCreateAlternative(models.TransientModel):
             )
         vals = self._get_alternative_values()
         alt_po = self.env['purchase.order'].with_context(origin_po_id=self.origin_po_id.id, default_requisition_id=False).create(vals)
+        alt_po.order_line._compute_tax_id()
         return {
             'type': 'ir.actions.act_window',
             'view_mode': 'form',