[FIX] purchase: correctly calculate average price

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#110740

Signed-off-by: William Henrotin (whe) <whe@odoo.com>

addons/purchase/report/purchase_report.py

@@ -168,10 +168,24 @@ class PurchaseReport(models.Model):
             if any(field.split(':')[1].split('(')[0] != 'avg' for field in [avg_days_to_purchase] if field):
                 raise UserError("Value: 'avg_days_to_purchase' should only be used to show an average. If you are seeing this message then it is being accessed incorrectly.")
 
+        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]
         if not res and avg_days_to_purchase:
             res = [{}]
 

addons/purchase/tests/test_purchase_order_report.py

@@ -88,3 +88,38 @@ class TestPurchaseOrderReport(AccountTestInvoicingCommon):
         )
         self.assertEqual(round(report[0]['delay']), -10, msg="The PO has been confirmed 10 days in advance")
         self.assertEqual(round(report[0]['delay_pass']), 5, msg="There are 5 days between the order date and the planned date")
+
+    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()
+        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)