Fix for #214

ormar/queryset/actions/filter_action.py

@@ -141,8 +141,10 @@ class FilterAction(QueryAction):
         op_attr = FILTER_OPERATORS[self.operator]
         if self.operator == "isnull":
             op_attr = "is_" if self.filter_value else "isnot"
-            self.filter_value = None
-        clause = getattr(self.column, op_attr)(self.filter_value)
+            filter_value = None
+        else:
+            filter_value = self.filter_value
+        clause = getattr(self.column, op_attr)(filter_value)
         clause = self._compile_clause(
             clause, modifiers={"escape": "\\" if self.has_escaped_character else None},
         )

tests/test_queries/test_isnull_filter.py

@@ -68,6 +68,13 @@ async def test_is_null():
         assert tolkien.books[0].year is None
         assert tolkien.books[0].title == "The Hobbit"
 
+        tolkien = await Author.objects.select_related("books").paginate(1, 10).get(
+            books__year__isnull=True
+        )
+        assert len(tolkien.books) == 1
+        assert tolkien.books[0].year is None
+        assert tolkien.books[0].title == "The Hobbit"
+
         tolkien = await Author.objects.select_related("books").get(
             books__year__isnull=False
         )