feat: big

cost/tests/test_models.py

@@ -142,4 +142,54 @@ class CostEntryModelTests(TestCase):
             occurred_at=date(2026, 6, 1),
         )
         self.assertFalse(bool(entry.image1))
-        self.assertFalse(bool(entry.image2))
+        self.assertFalse(bool(entry.image2))
+
+    def test_formula_amount_calculated_on_create(self):
+        entry = cost_models.CostEntry.objects.create(
+            merchant=self.merchant,
+            category=self.category,
+            amount=None,
+            unit_amount=Decimal('120.00'),
+            quantity=Decimal('2.5'),
+            unit_name='人天',
+            occurred_at=date(2026, 6, 1),
+        )
+        self.assertEqual(entry.amount, Decimal('300.00'))
+        self.assertEqual(entry.unit_name, '人天')
+
+    def test_formula_amount_recalculated_on_update(self):
+        entry = cost_models.CostEntry.objects.create(
+            merchant=self.merchant,
+            category=self.category,
+            amount=Decimal('300.00'),
+            unit_amount=Decimal('100.00'),
+            quantity=Decimal('3'),
+            occurred_at=date(2026, 6, 1),
+        )
+        entry.quantity = Decimal('4')
+        entry.amount = Decimal('999.00')
+        entry.save()
+        entry.refresh_from_db()
+        self.assertEqual(entry.amount, Decimal('400.00'))
+
+    def test_formula_partial_fields_raise_validation_error(self):
+        with self.assertRaises(ValidationError) as ctx:
+            cost_models.CostEntry.objects.create(
+                merchant=self.merchant,
+                category=self.category,
+                amount=None,
+                unit_amount=Decimal('100.00'),
+                quantity=None,
+                occurred_at=date(2026, 6, 1),
+            )
+        self.assertIn('quantity', ctx.exception.message_dict)
+
+    def test_manual_amount_required_without_formula(self):
+        with self.assertRaises(ValidationError) as ctx:
+            cost_models.CostEntry.objects.create(
+                merchant=self.merchant,
+                category=self.category,
+                amount=None,
+                occurred_at=date(2026, 6, 1),
+            )
+        self.assertIn('amount', ctx.exception.message_dict)