[FIX] purchase: correctly calculate average price

cherry-pick of a4904fb4889c16322494981f5a1be586683704bd

Steps to reproduce the bug:
- Create a storable product “P1”
    - costing method: avco
- Create a PO:
    - Add the product “P1”:
        - Line 1: Qty= 10, price= $50
        - Line 2: Qty=1, price= $10
    - Confirm the PO and receive the product

- Go to purchase → Reporting → Purchase Analysis

Problem:
The average price is incorrect, the current calculation is:

(50 + 10) / 2 = 30

The average should take into account the quantities purchased in each
line, And not simply the number of line, so the correct calculation
should be:

((10 * 50) + (10 * 1)) / 11 = 46.36

The SQL query is correct, it is when applying the read_group that the
calculation is incorrect, we should override it to make a personalized
calculation of the average.

opw-3136406

closes odoo/odoo#136097

X-original-commit: cd84549a3e5ea09d230dc1d02e05456a2372fd34
Signed-off-by: William Henrotin (whe) <whe@odoo.com>
Signed-off-by: Djamel Touati (otd) <otd@odoo.com>

addons/purchase/report/purchase_report.py

@@ -5,7 +5,7 @@
 # Please note that these reports are not multi-currency !!!
 #
 
-from odoo import fields, models
+from odoo import fields, models, api
 
 
 class PurchaseReport(models.Model):
@@ -147,3 +147,29 @@ class PurchaseReport(models.Model):
                 currency_table.rate
         """
         return group_by_str
+
+    @api.model
+    def read_group(self, domain, fields, groupby, offset=0, limit=None, orderby=False, lazy=True):
+        """
+        This is a hack to allow us to correctly calculate the average price of product.
+        """
+        if 'price_average:avg' in fields:
+            fields.extend(['aggregated_qty_ordered:array_agg(qty_ordered)'])
+            fields.extend(['aggregated_price_average:array_agg(price_average)'])
+
+        res = []
+        if fields:
+            res = super(PurchaseReport, self).read_group(domain, fields, groupby, offset=offset, limit=limit, orderby=orderby, lazy=lazy)
+
+        if 'price_average:avg' in fields:
+            qties = 'aggregated_qty_ordered'
+            special_field = 'aggregated_price_average'
+            for data in res:
+                if data[special_field] and data[qties]:
+                    total_unit_cost = sum(float(value) * float(qty) for value, qty in zip(data[special_field], data[qties]) if qty and value)
+                    total_qty_ordered = sum(float(qty) for qty in data[qties] if qty)
+                    data['price_average'] = (total_unit_cost / total_qty_ordered) if total_qty_ordered else 0
+                del data[special_field]
+                del data[qties]
+
+        return res

addons/purchase/tests/test_purchase_order_report.py

@@ -184,3 +184,37 @@ class TestPurchaseOrderReport(AccountTestInvoicingCommon):
         po_2.flush_model()
         report = self.env['purchase.report'].search([('product_id', "=", self.product_a.id)])
         self.assertEqual(report.currency_id, self.env.company.currency_id)
+
+    def test_avg_price_calculation(self):
+        """
+            Check that the average price is calculated based on the quantity ordered in each line
+            PO:
+                - 10 unit of product A -> price $50
+                - 1 unit of product A -> price $10
+            Total qty_ordered: 11
+            avergae price: 46.36 = ((10 * 50) + (10 * 1)) / 11
+        """
+        po = self.env['purchase.order'].create({
+            'partner_id': self.partner_a.id,
+            'order_line': [
+                (0, 0, {
+                    'product_id': self.product_a.id,
+                    'product_qty': 10.0,
+                    'price_unit': 50.0,
+                }),
+                (0, 0, {
+                    'product_id': self.product_a.id,
+                    'product_qty': 1.0,
+                    'price_unit': 10.0,
+                }),
+            ],
+        })
+        po.button_confirm()
+        po.flush_model()
+        report = self.env['purchase.report'].read_group(
+            [('product_id', '=', self.product_a.id)],
+            ['qty_ordered', 'price_average:avg'],
+            ['product_id'],
+        )
+        self.assertEqual(report[0]['qty_ordered'], 11)
+        self.assertEqual(round(report[0]['price_average'], 2), 46.36)