Add support for the '/p/:post_id' route on the client-side

app/assets/javascripts/discourse/routes/app-route-map.js.es6

@@ -5,6 +5,8 @@ export default function() {
 
   this.route('about', { path: '/about', resetNamespace: true });
 
+  this.route('post', { path: '/p/:id' });
+
   // Topic routes
   this.route('topic', { path: '/t/:slug/:id', resetNamespace: true }, function() {
     this.route('fromParams', { path: '/' });

app/assets/javascripts/discourse/routes/post.js.es6

@@ -0,0 +1,9 @@
+import { ajax } from "discourse/lib/ajax";
+
+export default Discourse.Route.extend({
+  beforeModel({ params }) {
+    return ajax(`/p/${params.post.id}`).then(t => {
+      this.transitionTo("topic.fromParamsNear", t.slug, t.id, t.current_post_number);
+    });
+  }
+});

app/serializers/topic_view_serializer.rb

@@ -50,6 +50,7 @@ class TopicViewSerializer < ApplicationSerializer
              :unpinned,
              :pinned,
              :details,
+             :current_post_number,
              :highest_post_number,
              :last_read_post_number,
              :last_read_post_id,
@@ -172,6 +173,14 @@ class TopicViewSerializer < ApplicationSerializer
     object.topic_user.present?
   end
 
+  def current_post_number
+    [object.post_number, object.highest_post_number].min
+  end
+
+  def include_current_post_number?
+    object.highest_post_number.present?
+  end
+
   def highest_post_number
     object.highest_post_number
   end