Add proper cache control for embedded visualizations

rcvis/settings.py

@@ -84,6 +84,7 @@
     # Order of the next 3 is important
     'django.middleware.cache.UpdateCacheMiddleware',
     'django.middleware.common.CommonMiddleware',
+    'django.middleware.http.ConditionalGetMiddleware',
     'django.middleware.cache.FetchFromCacheMiddleware',
 
     'django.middleware.csrf.CsrfViewMiddleware',

visualizer/migrations/0033_add_updated_at.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.28 on 2026-02-22 12:21
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('visualizer', '0032_jsonconfig_forcefirstrounddeterminespercentages'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='jsonconfig',
+            name='updatedAt',
+            field=models.DateTimeField(auto_now=True),
+        ),
+    ]

visualizer/models.py

@@ -50,6 +50,7 @@ class JsonConfig(models.Model):
     candidateSidecarFile = models.FileField(null=True, blank=True)
     slug = models.SlugField(unique=True, max_length=255)
     uploadedAt = models.DateTimeField(auto_now_add=True)
+    updatedAt = models.DateTimeField(auto_now=True)
     owner = models.ForeignKey(
         settings.AUTH_USER_MODEL,
         related_name='this_users_jsons',

visualizer/views.py

@@ -12,7 +12,7 @@
 from django.contrib.auth import get_user_model
 from django.contrib.auth.mixins import LoginRequiredMixin
 from django.core.cache import cache
-from django.http import JsonResponse, HttpResponse
+from django.http import JsonResponse, HttpResponse, HttpResponseNotModified
 from django.shortcuts import render
 from django.templatetags.static import static
 from django.urls import Resolver404
@@ -21,6 +21,8 @@
 from django.utils.decorators import method_decorator
 from django.views import View
 from django.views.decorators.clickjacking import xframe_options_exempt
+from django.utils.cache import patch_cache_control
+from django.utils.http import http_date, parse_http_date_safe
 from django.views.decorators.vary import vary_on_headers
 from django.views.generic.base import TemplateView, RedirectView
 from django.views.generic.detail import DetailView
@@ -160,8 +162,42 @@ def _actions_before_save(self, form):
         self.model.jsonFile.save('datatablesfile.json', form.cleaned_data['jsonFile'])
 
 
+class ConditionalGetMixin:
+    """
+    Mixin for DetailView subclasses that serve JsonConfig visualizations.
+    Short-circuits with 304 Not Modified when the client's If-Modified-Since
+    matches the object's updatedAt, skipping expensive graph computation
+    and template rendering. Also sets Last-Modified and Cache-Control on
+    all responses.
+    """
+
+    def get(self, request, *args, **kwargs):
+        # Fetch object once — setting self.object avoids a second DB query
+        # when super().get() calls get_object() internally.
+        self.object = self.get_object()
+
+        # Short-circuit: if the client has a fresh copy, return 304 without
+        # doing any of the expensive graph computation or template rendering.
+        if self.object.updatedAt:
+            last_modified = self.object.updatedAt.timestamp()
+            if_modified_since = request.META.get('HTTP_IF_MODIFIED_SINCE')
+            if if_modified_since:
+                if_modified_since = parse_http_date_safe(if_modified_since)
+                if if_modified_since is not None and last_modified <= if_modified_since:
+                    response = HttpResponseNotModified()
+                    response['Last-Modified'] = http_date(last_modified)
+                    patch_cache_control(response, no_cache=True, max_age=0)
+                    return response
+
+        response = super().get(request, *args, **kwargs)
+        if self.object.updatedAt:
+            response['Last-Modified'] = http_date(self.object.updatedAt.timestamp())
+        patch_cache_control(response, no_cache=True, max_age=0)
+        return response
+
+
 @method_decorator(vary_on_headers('increment',), name='get')
-class Visualize(DetailView):
+class Visualize(ConditionalGetMixin, DetailView):
     """ Visualizing a single JsonConfig """
     model = JsonConfig
     template_name = 'visualizer/visualize.html'
@@ -198,9 +234,8 @@ def get_context_data(self, **kwargs):
         return data
 
 
-@method_decorator(vary_on_headers('increment',), name='get')
 @method_decorator(xframe_options_exempt, name='dispatch')
-class VisualizeEmbedded(DetailView):
+class VisualizeEmbedded(ConditionalGetMixin, DetailView):
     """
     The embedded visualization, to be used in an iframe.
     """
@@ -260,7 +295,7 @@ def get_redirect_url(self, *args, **kwargs):
 
 @method_decorator(vary_on_headers('increment',), name='get')
 @method_decorator(xframe_options_exempt, name='dispatch')
-class VisualizeBallotpedia(DetailView):
+class VisualizeBallotpedia(ConditionalGetMixin, DetailView):
     """ The embedded ballotpedia visualization """
     model = JsonConfig
     template_name = 'visualizer/visualize-ballotpedia.html'