Fix 'get_required_quantity' method (#13002)

- Ensure a Decimal is returned in all cases
- Fix type hinting
- Updated unit tests

src/backend/InvenTree/part/models.py

@@ -4316,11 +4316,11 @@ class BomItem(InvenTree.models.MetadataMixin, InvenTree.models.InvenTreeModel):
 
         return int(Decimal(available_stock) / n)
 
-    def get_required_quantity(self, build_quantity: float) -> float:
+    def get_required_quantity(self, build_quantity: Decimal | float | int) -> Decimal:
         """Calculate the required part quantity, based on the supplied build_quantity.
 
         Arguments:
-            build_quantity: Number of assemblies to build
+            build_quantity: Number of assemblies to build (converted to Decimal)
 
         Returns:
             Production quantity required for this component
@@ -4330,6 +4330,10 @@ class BomItem(InvenTree.models.MetadataMixin, InvenTree.models.InvenTreeModel):
             and piece_count indicates how many pieces are needed.
             Total material = quantity x piece_count x build_quantity.
         """
+        # Convert via str to avoid binary float artefacts (e.g. 0.1 -> 0.1000000000000000055...)
+        if not isinstance(build_quantity, Decimal):
+            build_quantity = Decimal(str(build_quantity))
+
         # Base quantity requirement (quantity is per-piece, piece_count is number of pieces)
         required = self.quantity * self.piece_count * build_quantity
 
@@ -4356,8 +4360,7 @@ class BomItem(InvenTree.models.MetadataMixin, InvenTree.models.InvenTreeModel):
             try:
                 round_up = Decimal(self.rounding_multiple)
                 value = Decimal(required)
-                value = math.ceil(value / round_up) * round_up
-                required = float(value)
+                required = math.ceil(value / round_up) * round_up
             except InvalidOperation:
                 log_error('bom_item.get_required_quantity')
 

src/backend/InvenTree/part/test_bom_item.py

@@ -1,6 +1,7 @@
 """Unit tests for the BomItem model."""
 
 import hashlib
+from decimal import Decimal
 from unittest import mock
 
 import django.core.exceptions as django_exceptions
@@ -112,6 +113,20 @@ class BomItemTest(TestCase):
         self.assertEqual(item.get_required_quantity(1), 8)  # 3 + 5 = 8
         self.assertEqual(item.get_required_quantity(10), 35)  # 30 + 5 = 35
 
+    def test_required_quantity_input_types(self):
+        """Test that get_required_quantity accepts int, float and Decimal inputs."""
+        item = BomItem.objects.get(pk=4)
+
+        for value, expected in [
+            (10, Decimal('30')),
+            (Decimal('2.5'), Decimal('7.5')),
+            (2.5, Decimal('7.5')),
+            (0.1, Decimal('0.3')),
+        ]:
+            result = item.get_required_quantity(value)
+            self.assertIsInstance(result, Decimal)
+            self.assertEqual(result, expected)
+
     def test_round_up(self):
         """Test the 'rounding_multiple' attribute."""
         item = BomItem.objects.get(pk=4)
@@ -143,6 +158,9 @@ class BomItemTest(TestCase):
             item.get_required_quantity(100), 306
         )  # 3 x 100 = 300, rounded up to nearest multiple of 17
 
+        # The rounded value must remain a Decimal (not a float)
+        self.assertIsInstance(item.get_required_quantity(10), Decimal)
+
         # Next, let's create a new Build order
         bo = build.models.Build.objects.create(
             part=item.part, quantity=21, reference='BO-9999', title='Test Build Order'