Use a local URL for image previews

wagtail/wagtailimages/static/wagtailimages/js/image-url-generator.js

@@ -47,7 +47,7 @@ $(function() {
             $.getJSON(generatorUrl.replace('__filterspec__', filterSpec))
                 .done(function(data) {
                     $result.val(data['url']);
-                    $preview.attr('src', data['url']);
+                    $preview.attr('src', data['local_url']);
                     $loadingMask.removeClass('loading');
                 })
                 .fail(function(data) {

wagtail/wagtailimages/tests.py

@@ -831,7 +831,7 @@ class TestGenerateURLView(TestCase, WagtailTestUtils):
         # Check JSON
         content_json = json.loads(response.content.decode())
 
-        self.assertEqual(list(content_json.keys()), ['url'])
+        self.assertEqual(set(content_json.keys()), set(['url', 'local_url']))
 
         expected_url = 'http://localhost/images/%(signature)s/%(image_id)d/fill-800x600/' % {
             'signature': urlquote(generate_signature(self.image.id, 'fill-800x600').decode()),
@@ -839,6 +839,12 @@ class TestGenerateURLView(TestCase, WagtailTestUtils):
         }
         self.assertEqual(content_json['url'], expected_url)
 
+        expected_local_url = '/images/%(signature)s/%(image_id)d/fill-800x600/' % {
+            'signature': urlquote(generate_signature(self.image.id, 'fill-800x600').decode()),
+            'image_id': self.image.id,
+        }
+        self.assertEqual(content_json['local_url'], expected_local_url)
+
     def test_get_bad_permissions(self):
         """
         This tests that the view gives a 403 if a user without correct permissions attemts to access it

wagtail/wagtailimages/views/images.py

@@ -168,11 +168,13 @@ def generate_url(request, image_id, filter_spec):
     signature = generate_signature(image_id, filter_spec)
     url = reverse('wagtailimages_serve', args=(signature, image_id, filter_spec))
 
-    # Add default sites root URL to the beginning
-    site_root_url = Site.objects.get(is_default_site=True).root_url
-    url = site_root_url + url
+    # Get site root url
+    try:
+        site_root_url = Site.objects.get(is_default_site=True).root_url
+    except Site.DoesNotExist:
+        site_root_url = Site.objects.first().root_url
 
-    return json_response({'url': url}, status=200)
+    return json_response({'url': site_root_url + url, 'local_url': url}, status=200)
 
 
 @permission_required('wagtailadmin.access_admin')  # more specific permission tests are applied within the view