Sort by id, not `created`, when pruning pings.

hc/api/models.py

@@ -104,7 +104,7 @@ class Check(models.Model):
 
         """
 
-        pings = Ping.objects.filter(owner=self).order_by("-created")
+        pings = Ping.objects.filter(owner=self).order_by("-id")
         cutoff = pings[keep_limit:keep_limit+1]
 
         # If cutoff is empty slice then the check has less than `keep_limit`

hc/api/tests/test_check_model.py

@@ -9,7 +9,7 @@ class CheckModelTestCase(TestCase):
         check.save()
 
         for i in range(0, 6):
-            p = Ping(pk=100 + i, owner=check, ua="UA%d" % i)
+            p = Ping(owner=check, ua="UA%d" % i)
             p.save()
 
         check.prune_pings(keep_limit=3)