PERF: Only send down suggested payload when loading last chunk.

app/assets/javascripts/discourse/models/post-stream.js.es6

@@ -741,6 +741,11 @@ export default RestModel.extend({
     const store = this.store;
     return ajax(url, {data}).then(result => {
       const posts = Ember.get(result, "post_stream.posts");
+
+      if (result.suggested_topics) {
+        this.set('topic.suggested_topics', result.suggested_topics);
+      }
+
       if (posts) {
         posts.forEach(p => this.storePost(store.createRecord('post', p)));
       }

app/assets/javascripts/discourse/models/topic-details.js.es6

@@ -18,13 +18,6 @@ const TopicDetails = RestModel.extend({
       });
     }
 
-    if (details.suggested_topics) {
-      const store = this.store;
-      details.suggested_topics = details.suggested_topics.map(function (st) {
-        return store.createRecord('topic', st);
-      });
-    }
-
     if (details.participants) {
       details.participants = details.participants.map(function (p) {
         p.topic = topic;

app/assets/javascripts/discourse/models/topic.js.es6

@@ -99,6 +99,17 @@ const Topic = RestModel.extend({
     return newTags;
   },
 
+  @computed("suggested_topics")
+  suggestedTopics(suggestedTopics) {
+    if (suggestedTopics) {
+      const store = this.store;
+
+      return this.set('suggested_topics', suggestedTopics.map(st => {
+        return store.createRecord('topic', st);
+      }));
+    }
+  },
+
   replyCount: function() {
     return this.get('posts_count') - 1;
   }.property('posts_count'),

app/assets/javascripts/discourse/templates/components/suggested-topics.hbs

@@ -4,9 +4,9 @@
     {{basic-topic-list
       hideCategory="true"
       showPosters="true"
-      topics=topic.details.suggested_topics}}
+      topics=topic.suggestedTopics}}
   {{else}}
-    {{basic-topic-list topics=topic.details.suggested_topics}}
+    {{basic-topic-list topics=topic.suggestedTopics}}
   {{/if}}
 </div>
 <h3 class='suggested-topics-message'>{{{browseMoreMessage}}}</h3>

app/assets/javascripts/discourse/templates/topic.hbs

@@ -245,7 +245,7 @@
 
               {{plugin-outlet name="topic-above-suggested" args=(hash model=model)}}
 
-              {{#if model.details.suggested_topics.length}}
+              {{#if model.suggestedTopics.length}}
                 {{suggested-topics topic=model}}
               {{/if}}
             {{/if}}

app/controllers/topics_controller.rb

@@ -717,7 +717,12 @@ class TopicsController < ApplicationController
       return
     end
 
-    topic_view_serializer = TopicViewSerializer.new(@topic_view, scope: guardian, root: false, include_raw: !!params[:include_raw])
+    topic_view_serializer = TopicViewSerializer.new(@topic_view,
+      scope: guardian,
+      root: false,
+      include_raw: !!params[:include_raw],
+      page: params[:page]
+    )
 
     respond_to do |format|
       format.html do

app/serializers/suggested_topics_mixin.rb

@@ -0,0 +1,15 @@
+module SuggestedTopicsMixin
+  def self.included(klass)
+    klass.attributes :suggested_topics
+  end
+
+  def include_suggested_topics?
+    object.next_page.nil? && object.suggested_topics&.topics.present?
+  end
+
+  def suggested_topics
+    object.suggested_topics.topics.map do |t|
+      SuggestedTopicSerializer.new(t, scope: scope, root: false)
+    end
+  end
+end

app/serializers/topic_view_posts_serializer.rb

@@ -1,5 +1,6 @@
 class TopicViewPostsSerializer < ApplicationSerializer
   include PostStreamSerializerMixin
+  include SuggestedTopicsMixin
 
   attributes :id
 

app/serializers/topic_view_serializer.rb

@@ -3,6 +3,7 @@ require_dependency 'new_post_manager'
 
 class TopicViewSerializer < ApplicationSerializer
   include PostStreamSerializerMixin
+  include SuggestedTopicsMixin
   include ApplicationHelper
 
   def self.attributes_from_topic(*list)
@@ -94,12 +95,6 @@ class TopicViewSerializer < ApplicationSerializer
       end
     end
 
-    if object.suggested_topics&.topics.present?
-      result[:suggested_topics] = object.suggested_topics.topics.map do |t|
-        SuggestedTopicSerializer.new(t, scope: scope, root: false)
-      end
-    end
-
     if object.links.present?
       result[:links] = object.links.map do |user|
         TopicLinkSerializer.new(user, scope: scope, root: false)

spec/serializers/topic_view_serializer_spec.rb

@@ -0,0 +1,38 @@
+require 'rails_helper'
+
+describe TopicViewSerializer do
+  let(:topic) { Fabricate(:topic) }
+  let(:user) { Fabricate(:user) }
+
+  describe '#suggested_topics' do
+    let(:topic2) { Fabricate(:topic) }
+
+    before do
+      TopicUser.update_last_read(user, topic2.id, 0, 0)
+    end
+
+    describe 'when loading last chunk' do
+      it 'should include suggested topics' do
+        topic_view = TopicView.new(topic.id, user)
+        json = described_class.new(topic_view, scope: Guardian.new(user), root: false).as_json
+
+        expect(json[:suggested_topics].first.id).to eq(topic2.id)
+      end
+    end
+
+    describe 'when not loading last chunk' do
+      let(:post) { Fabricate(:post, topic: topic) }
+      let(:post2) { Fabricate(:post, topic: topic) }
+
+      it 'should not include suggested topics' do
+        post
+        post2
+        topic_view = TopicView.new(topic.id, user, post_ids: [post.id])
+        topic_view.next_page
+        json = described_class.new(topic_view, scope: Guardian.new(user), root: false).as_json
+
+        expect(json[:suggested_topics]).to eq(nil)
+      end
+    end
+  end
+end