Asymmetry between exact and iexact when filtering for empty strings on Oracle

[Clifford Gama]

​As pointed out by Jacob, the special-handling that the ORM does for empty strings on backends that interpret_empty_strings_as_null=True is not performed on iexact lookups. The ​docs on iexact and exact state that:

If the value provided for comparison is None, it will be interpreted as an SQL NULL (see isnull for more details).

Which I would also expect to apply to empty strings on backends where they mean Null.

Failing test:

diff --git a/tests/queries/tests.py b/tests/queries/tests.py
index 51d1915c97..24e9c51593 100644
--- a/tests/queries/tests.py
+++ b/tests/queries/tests.py
@@ -2280,6 +2280,17 @@ class ComparisonTests(TestCase):
            [item_ab],
        )

+    @skipUnlessDBFeature("interprets_empty_strings_as_nulls")
+    def test_empty_string_is_null(self):
+        obj = NullableName.objects.create(name=None)
+        obj1 = NullableName.objects.create(name="")
+        cases = [{"name__exact": ""}, {"name__iexact": ""}]
+        for lookup in cases:
+            with self.subTest(lookup):
+                self.assertSequenceEqual(
+                    NullableName.objects.filter(**lookup), [obj, obj1]
+                )
+

class ExistsSql(TestCase):
    def test_exists(self):

The query generated for the iexact case is as follows (note that it will never match anything):

SELECT "queries_nullablename"."id",
       "queries_nullablename"."name"
FROM   "queries_nullablename"
WHERE  Upper("queries_nullablename"."name") = Upper()
ORDER  BY "queries_nullablename"."id" ASC;  

[Jacob Walls]

Thanks, Clifford!