[1.10.x] Fixed #27482 -- Doc'd an example of Case() in QuerySet.filter().

Backport of b28c6ca763 from master

docs/ref/models/conditional-expressions.txt

@@ -141,6 +141,20 @@ the ``Client`` has been with us, we could do so using lookups::
     both Jane Doe and Jack Black. This works just like an :keyword:`if` ...
     :keyword:`elif` ... :keyword:`else` statement in ``Python``.
 
+``Case()`` also works in a ``filter()`` clause. For example, to find gold
+clients that registered more than a month ago and platinum clients that
+registered more than a year ago::
+
+    >>> a_month_ago = date.today() - timedelta(days=30)
+    >>> a_year_ago = date.today() - timedelta(days=365)
+    >>> Client.objects.filter(
+    ...     registered_on__lte=Case(
+    ...         When(account_type=Client.GOLD, then=a_month_ago),
+    ...         When(account_type=Client.PLATINUM, then=a_year_ago),
+    ...     ),
+    ... ).values_list('name', 'account_type')
+    [('Jack Black', 'P')]
+
 Advanced queries
 ================
 

tests/expressions_case/tests.py

@@ -1279,3 +1279,17 @@ class CaseDocumentationExamples(TestCase):
             ),
             {'regular': 2, 'gold': 1, 'platinum': 3}
         )
+
+    def test_filter_example(self):
+        a_month_ago = date.today() - timedelta(days=30)
+        a_year_ago = date.today() - timedelta(days=365)
+        self.assertQuerysetEqual(
+            Client.objects.filter(
+                registered_on__lte=Case(
+                    When(account_type=Client.GOLD, then=a_month_ago),
+                    When(account_type=Client.PLATINUM, then=a_year_ago),
+                ),
+            ),
+            [('Jack Black', 'P')],
+            transform=attrgetter('name', 'account_type')
+        )