Fix TypeError raised when comparing nullable property (#4279)

* Fix `TypeError` raised when comparing nullable property

* Fix failing lint tests

src/oscar/apps/order/abstract_models.py

@@ -883,7 +883,7 @@ def is_allocation_consumption_possible(self, quantity):
         if self.allocation_cancelled:
             return False
 
-        return quantity <= self.num_allocated
+        return quantity <= (self.num_allocated or 0)
 
     def consume_allocation(self, quantity):
         if not self.can_track_allocations:

src/oscar/apps/partner/abstract_models.py

@@ -222,7 +222,7 @@ def is_allocation_consumption_possible(self, quantity):
         """
         Test if a proposed stock consumption is permitted
         """
-        return quantity <= min(self.num_allocated, self.num_in_stock)
+        return quantity <= min(self.num_allocated or 0, self.num_in_stock)
 
     def consume_allocation(self, quantity):
         """

tests/integration/order/test_models.py

@@ -166,6 +166,40 @@ def setUp(self):
     def tearDown(self):
         ShippingEventType.objects.all().delete()
 
+    def test_is_allocation_consumption_possible_when_num_allocated_is_greater_than_quantity(
+        self,
+    ):
+        self.line.num_allocated = 2
+
+        actual = self.line.is_allocation_consumption_possible(1)
+
+        self.assertTrue(actual)
+
+    def test_is_allocation_consumption_possible_when_num_allocated_is_lower_than_quantity(
+        self,
+    ):
+        self.line.num_allocated = 0
+
+        actual = self.line.is_allocation_consumption_possible(1)
+
+        self.assertFalse(actual)
+
+    def test_is_allocation_consumption_possible_when_num_allocated_is_equal_to_quantity(
+        self,
+    ):
+        self.line.num_allocated = 1
+
+        actual = self.line.is_allocation_consumption_possible(1)
+
+        self.assertTrue(actual)
+
+    def test_is_allocation_consumption_possible_when_num_allocated_is_null(self):
+        self.line.num_allocated = None
+
+        actual = self.line.is_allocation_consumption_possible(1)
+
+        self.assertFalse(actual)
+
     def event(self, event_type, quantity=None):
         """
         Creates a shipping event for the test line

tests/integration/partner/test_models.py

@@ -17,6 +17,40 @@ def setUp(self):
             self.product, price=D("10.00"), num_in_stock=10
         )
 
+    def test_is_allocation_consumption_possible_when_num_allocated_is_greater_than_quantity(
+        self,
+    ):
+        self.stockrecord.num_allocated = 2
+
+        actual = self.stockrecord.is_allocation_consumption_possible(1)
+
+        self.assertTrue(actual)
+
+    def test_is_allocation_consumption_possible_when_num_allocated_is_lower_than_quantity(
+        self,
+    ):
+        self.stockrecord.num_allocated = 0
+
+        actual = self.stockrecord.is_allocation_consumption_possible(1)
+
+        self.assertFalse(actual)
+
+    def test_is_allocation_consumption_possible_when_num_allocated_is_equal_to_quantity(
+        self,
+    ):
+        self.stockrecord.num_allocated = 1
+
+        actual = self.stockrecord.is_allocation_consumption_possible(1)
+
+        self.assertTrue(actual)
+
+    def test_is_allocation_consumption_possible_when_num_allocated_is_null(self):
+        self.stockrecord.num_allocated = None
+
+        actual = self.stockrecord.is_allocation_consumption_possible(1)
+
+        self.assertFalse(actual)
+
     def test_get_price_excl_tax_returns_correct_value(self):
         self.assertEqual(D("10.00"), self.stockrecord.price)