Fix backend template rendering

rest_framework_filters/backends.py

@@ -1,5 +1,5 @@
 
-from django.template import loader
+from django.template import Template, TemplateDoesNotExist, loader
 from rest_framework import compat
 from django_filters.rest_framework import backends
 
@@ -28,8 +28,11 @@ def to_html(self, request, queryset, view):
         # forces `form` evaluation before `qs` is called. This prevents an empty form from being cached.
         filter_instance.form
 
-        context = {
+        try:
+            template = loader.get_template(self.template)
+        except TemplateDoesNotExist:
+            template = Template(backends.template_default)
+
+        return compat.template_render(template, context={
             'filter': filter_instance
-        }
-        template = loader.get_template(self.template)
-        return compat.template_render(template, context)
+        })

tests/test_backends.py

@@ -1,8 +1,10 @@
 
-from rest_framework.test import APITestCase
+from rest_framework.test import APITestCase, APIRequestFactory
 
 from .testapp import models, views
 
+factory = APIRequestFactory()
+
 
 class BackendTest(APITestCase):
 
@@ -33,3 +35,27 @@ def test_filter_fields_reusability(self):
         self.assertEqual(len(response.data), 1)
         self.assertEqual(response.data[0]['username'], 'user1')
         self.assertDictEqual(views.FilterFieldsUserViewSet.filter_fields, {'username': '__all__'})
+
+    def test_backend_output_sanity(self):
+        """
+        Sanity check to ensure backend can at least render something without crashing.
+        """
+        class SimpleViewSet(views.FilterFieldsUserViewSet):
+            filter_fields = ['username']
+
+        view = SimpleViewSet(action_map={})
+        backend = view.filter_backends[0]
+        request = view.initialize_request(factory.get('/'))
+        html = backend().to_html(request, view.get_queryset(), view)
+
+        self.assertHTMLEqual(html, """
+        <h2>Field filters</h2>
+        <form class="form" action="" method="get">
+            <p>
+                <label for="id_username">Username:</label>
+                <input id="id_username" name="username" type="text" />
+                <span class="helptext">Filter</span>
+            </p>
+            <button type="submit" class="btn btn-primary">Submit</button>
+        </form>
+        """)