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

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