[fix] Admin: fixed template ordering bug in preview config

The order of templates was not always retained when
generating the preview of a config object.

Author: nemesifier

openwisp_controller/config/admin.py

@@ -230,11 +230,14 @@ def preview_view(self, request):
         template_ids = request.POST.get('templates')
         if template_ids:
             template_model = config_model.get_template_model()
+            template_ids = template_ids.split(',')
             try:
-                templates = template_model.objects.filter(
-                    pk__in=template_ids.split(',')
-                )
+                templates = template_model.objects.filter(pk__in=template_ids)
                 templates = list(templates)  # evaluating queryset performs query
+                # ensure the order of templates is maintained
+                templates.sort(
+                    key=lambda template: template_ids.index(str(template.id))
+                )
             except ValidationError as e:
                 logger.exception(error_msg, extra={'request': request})
                 return HttpResponse(str(e), status=400)

openwisp_controller/config/tests/test_admin.py

@@ -701,10 +701,17 @@ def test_preview_device_jsonerror(self):
 
     def test_preview_device_showerror(self):
         t1 = Template.objects.get(name='dhcp')
-        t2 = Template(name='t', config=t1.config, backend='netjsonconfig.OpenWrt')
-        t2.full_clean()
-        t2.save()
-        templates = [t1, t2]
+        t2 = Template(name='t2', config=t1.config, backend=t1.backend)
+        t3 = Template(name='t3', config=t1.config, backend=t1.backend)
+        t4 = Template(
+            name='t4',
+            config={"interfaces": [{"name": "eth0", "type": "bridge", "stp": "WRONG"}]},
+            backend='netjsonconfig.OpenWrt',
+        )
+        # skip validating config to raise error later
+        t4.save()
+        # adding multiple templates to ensure the order is retained correctly
+        templates = [t1, t2, t3, t4]
         path = reverse(f'admin:{self.app_label}_device_preview')
         data = {
             'name': 'test-device',