[1.8.x] Fixed #25772 -- Corrected __len lookup on ArrayField for empty arrays.

Backport of 88fc9e2826 from master

django/contrib/postgres/fields/array.py

@@ -198,7 +198,11 @@ class ArrayLenTransform(Transform):
 
     def as_sql(self, compiler, connection):
         lhs, params = compiler.compile(self.lhs)
-        return 'array_length(%s, 1)' % lhs, params
+        # Distinguish NULL and empty arrays
+        return (
+            'CASE WHEN %(lhs)s IS NULL THEN NULL ELSE '
+            'coalesce(array_length(%(lhs)s, 1), 0) END'
+        ) % {'lhs': lhs}, params
 
 
 class IndexTransform(Transform):

docs/releases/1.8.8.txt

@@ -11,3 +11,6 @@ Bugfixes
 
 * Fixed incorrect ``unique_together`` field name generation by ``inspectdb``
   (:ticket:`25274`).
+
+* Corrected ``__len`` query lookup on ``ArrayField`` for empty arrays
+  (:ticket:`25772`).

tests/postgres_tests/test_array.py

@@ -218,6 +218,13 @@ class TestQuerying(TestCase):
             self.objs[0:3]
         )
 
+    def test_len_empty_array(self):
+        obj = NullableIntegerArrayModel.objects.create(field=[])
+        self.assertSequenceEqual(
+            NullableIntegerArrayModel.objects.filter(field__len=0),
+            [obj]
+        )
+
     def test_slice(self):
         self.assertSequenceEqual(
             NullableIntegerArrayModel.objects.filter(field__0_1=[2]),