Calculate weighted average price when merging stock items (#7534)

* Calculate weighted average price when merging stock items

* refactor currency averaging

- Only add samples which have an associated value

* Revert to using two loops

* Check for div-by-zero

* Add unit testing for purchase price averaging

src/backend/InvenTree/stock/models.py

@@ -20,6 +20,7 @@ from django.dispatch import receiver
 from django.urls import reverse
 from django.utils.translation import gettext_lazy as _
 
+from djmoney.contrib.exchange.models import convert_money
 from mptt.managers import TreeManager
 from mptt.models import MPTTModel, TreeForeignKey
 from taggit.managers import TaggableManager
@@ -1706,6 +1707,12 @@ class StockItem(
 
         parent_id = self.parent.pk if self.parent else None
 
+        # Keep track of pricing data for the merged data
+        pricing_data = []
+
+        if self.purchase_price:
+            pricing_data.append([self.purchase_price, self.quantity])
+
         for other in other_items:
             # If the stock item cannot be merged, return
             if not self.can_merge(other, raise_error=raise_error, **kwargs):
@@ -1714,11 +1721,15 @@ class StockItem(
                 )
                 return
 
+        for other in other_items:
             tree_ids.add(other.tree_id)
 
-        for other in other_items:
             self.quantity += other.quantity
 
+            if other.purchase_price:
+                # Only add pricing data if it is available
+                pricing_data.append([other.purchase_price, other.quantity])
+
             # Any "build order allocations" for the other item must be assigned to this one
             for allocation in other.allocations.all():
                 allocation.stock_item = self
@@ -1744,7 +1755,31 @@ class StockItem(
             deltas={'location': location.pk if location else None},
         )
 
+        # Update the location of the item
         self.location = location
+
+        # Update the unit price - calculate weighted average of available pricing data
+        if len(pricing_data) > 0:
+            unit_price, quantity = pricing_data[0]
+
+            # Use the first currency as the base currency
+            base_currency = unit_price.currency
+
+            total_price = unit_price * quantity
+
+            for price, qty in pricing_data[1:]:
+                # Attempt to convert the price to the base currency
+                try:
+                    price = convert_money(price, base_currency)
+                    total_price += price * qty
+                    quantity += qty
+                except:
+                    # Skip this entry, cannot convert to base currency
+                    continue
+
+            if quantity > 0:
+                self.purchase_price = total_price / quantity
+
         self.save()
 
         # Rebuild stock trees as required

src/backend/InvenTree/stock/tests.py

@@ -755,23 +755,14 @@ class StockTest(StockTestBase):
         # First, we will create a stock location structure
 
         A = StockLocation.objects.create(name='A', description='Top level location')
-
         B1 = StockLocation.objects.create(name='B1', parent=A)
-
         B2 = StockLocation.objects.create(name='B2', parent=A)
-
         B3 = StockLocation.objects.create(name='B3', parent=A)
-
         C11 = StockLocation.objects.create(name='C11', parent=B1)
-
         C12 = StockLocation.objects.create(name='C12', parent=B1)
-
         C21 = StockLocation.objects.create(name='C21', parent=B2)
-
         C22 = StockLocation.objects.create(name='C22', parent=B2)
-
         C31 = StockLocation.objects.create(name='C31', parent=B3)
-
         C32 = StockLocation.objects.create(name='C32', parent=B3)
 
         # Check that the tree_id is correct for each sublocation
@@ -895,6 +886,62 @@ class StockTest(StockTestBase):
 
             self.assertEqual(len(p.metadata.keys()), 4)
 
+    def test_merge(self):
+        """Test merging of multiple stock items."""
+        from djmoney.money import Money
+
+        part = Part.objects.first()
+        part.stock_items.all().delete()
+
+        # Test simple merge without any pricing information
+        s1 = StockItem.objects.create(part=part, quantity=10)
+        s2 = StockItem.objects.create(part=part, quantity=20)
+        s3 = StockItem.objects.create(part=part, quantity=30)
+
+        self.assertEqual(part.stock_items.count(), 3)
+        s1.merge_stock_items([s2, s3])
+        self.assertEqual(part.stock_items.count(), 1)
+        s1.refresh_from_db()
+        self.assertEqual(s1.quantity, 60)
+        self.assertIsNone(s1.purchase_price)
+
+        part.stock_items.all().delete()
+
+        # Create some stock items with pricing information
+        s1 = StockItem.objects.create(part=part, quantity=10, purchase_price=None)
+        s2 = StockItem.objects.create(
+            part=part, quantity=15, purchase_price=Money(10, 'USD')
+        )
+        s3 = StockItem.objects.create(part=part, quantity=30)
+
+        self.assertEqual(part.stock_items.count(), 3)
+        s1.merge_stock_items([s2, s3])
+        self.assertEqual(part.stock_items.count(), 1)
+        s1.refresh_from_db()
+        self.assertEqual(s1.quantity, 55)
+        self.assertEqual(s1.purchase_price, Money(10, 'USD'))
+
+        part.stock_items.all().delete()
+
+        s1 = StockItem.objects.create(
+            part=part, quantity=10, purchase_price=Money(5, 'USD')
+        )
+        s2 = StockItem.objects.create(
+            part=part, quantity=25, purchase_price=Money(10, 'USD')
+        )
+        s3 = StockItem.objects.create(
+            part=part, quantity=5, purchase_price=Money(75, 'USD')
+        )
+
+        self.assertEqual(part.stock_items.count(), 3)
+        s1.merge_stock_items([s2, s3])
+        self.assertEqual(part.stock_items.count(), 1)
+        s1.refresh_from_db()
+        self.assertEqual(s1.quantity, 40)
+
+        # Final purchase price should be the weighted average
+        self.assertAlmostEqual(s1.purchase_price.amount, 16.875, places=3)
+
 
 class StockBarcodeTest(StockTestBase):
     """Run barcode tests for the stock app."""