Test for check.prune_pings() method

hc/api/models.py

@@ -112,8 +112,8 @@ class Check(models.Model):
         if len(cutoff) == 0:
             return 0
 
-        cutoff_date = cutoff[0].created
-        q = Ping.objects.filter(owner=self, created__lte=cutoff_date)
+        cutoff_id = cutoff[0].id
+        q = Ping.objects.filter(owner=self, id__lte=cutoff_id)
         n_pruned, _ = q.delete()
 
         self.n_pings = keep_limit

hc/api/tests/test_check_model.py

@@ -0,0 +1,19 @@
+from django.test import TestCase
+from hc.api.models import Check, Ping
+
+
+class CheckModelTestCase(TestCase):
+
+    def test_prune_pings(self):
+        check = Check()
+        check.save()
+
+        for i in range(0, 6):
+            p = Ping(owner=check, ua="UA%d" % i)
+            p.save()
+
+        check.prune_pings(keep_limit=3)
+
+        ua_set = set(Ping.objects.values_list("ua", flat=True))
+        # UA0, UA1, UA2 should have been pruned--
+        self.assertEqual(ua_set, set(["UA3", "UA4", "UA5"]))