Bug fix for purchase order pricing (#4373)

* Account for pack size when calculating purchase_price for received line items

* Clearer display of "unit pricing" when part has units

* Add data migration to fix historical pricing bugs

* Remove debug statement

InvenTree/stock/migrations/0094_auto_20230220_0025.py

@@ -0,0 +1,79 @@
+# Generated by Django 3.2.18 on 2023-02-20 00:25
+
+import logging
+
+from django.db import migrations
+
+logger = logging.getLogger('inventree')
+
+
+def fix_purchase_price(apps, schema_editor):
+    """Data migration for fixing historical issue with StockItem.purchase_price field.
+
+    Ref: https://github.com/inventree/InvenTree/pull/4373
+
+    Due to an existing bug, if a PurchaseOrderLineItem was received,
+    which had:
+
+    a) A SupplierPart with a non-unity pack size
+    b) A defined purchase_price
+
+    then the StockItem.purchase_price was not calculated correctly!
+
+    Specifically, the purchase_price was not divided through by the pack_size attribute.
+
+    This migration fixes this by looking through all stock items which:
+
+    - Is linked to a purchase order
+    - Have a purchase_price field
+    - Are linked to a supplier_part
+    - We can determine correctly that the calculation was misapplied
+    """
+
+    StockItem = apps.get_model('stock', 'stockitem')
+
+    items = StockItem.objects.exclude(
+        purchase_order=None
+    ).exclude(
+        supplier_part=None
+    ).exclude(
+        purchase_price=None
+    ).exclude(
+        supplier_part__pack_size=1
+    )
+
+    n_updated = 0
+
+    for item in items:
+        # Grab a reference to the associated PurchaseOrder
+        # Trying to find an absolute match between this StockItem and an associated PurchaseOrderLineItem
+        po = item.purchase_order
+        for line in po.lines.all():
+            # SupplierPart match
+            if line.part == item.supplier_part:
+                # Unit price matches original PurchaseOrder (and is thus incorrect)
+                if item.purchase_price == line.purchase_price:
+                    item.purchase_price /= item.supplier_part.pack_size
+                    item.save()
+
+                    n_updated += 1
+
+    if n_updated > 0:
+        logger.info(f"Corrected purchase_price field for {n_updated} stock items.")
+
+
+def reverse(apps, schema_editor):  # pragmae: no cover
+    pass
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0093_auto_20230217_2140'),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            fix_purchase_price,
+            reverse_code=reverse,
+        )
+    ]

InvenTree/stock/templates/stock/item_base.html

@@ -187,7 +187,10 @@
     <tr>
         <td><span class='fas fa-dollar-sign'></span></td>
         <td>{% trans "Purchase Price" %}</td>
-        <td>{% include "price_data.html" with price=item.purchase_price %}</td>
+        <td>
+            {% include "price_data.html" with price=item.purchase_price %}
+            {% if item.part.units %} / {{ item.part.units }}{% endif %}
+        </td>
     </tr>
     {% endif %}
     {% if item.parent %}