PERF: do not include suggested topics when loading new posts

When a new post is triggered via message bus post stream will attempt to load
it, previously the `/topic/TOPIC_ID/posts.json` would unconditionally include
suggested topics, this caused excessive load on the server.

New pattern defaults to exclude suggested and related topics from this API
unless people explicitly ask for suggested.

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

@@ -907,10 +907,13 @@ export default RestModel.extend({
   },
 
   fetchNextWindow(postNumber, asc, callback) {
+    let includeSuggested = !this.get("topic.suggested_topics");
+
     const url = `/t/${this.get("topic.id")}/posts.json`;
     let data = {
       post_number: postNumber,
-      asc: asc
+      asc: asc,
+      include_suggested: includeSuggested
     };
 
     data = _.merge(data, this.get("streamFilters"));
@@ -950,8 +953,10 @@ export default RestModel.extend({
       return Ember.RSVP.resolve([]);
     }
 
+    let includeSuggested = !this.get("topic.suggested_topics");
+
     const url = "/t/" + this.get("topic.id") + "/posts.json";
-    const data = { post_ids: postIds };
+    const data = { post_ids: postIds, include_suggested: includeSuggested };
     const store = this.store;
 
     return ajax(url, { data }).then(result => {

app/controllers/topics_controller.rb

@@ -197,13 +197,17 @@ class TopicsController < ApplicationController
 
   def posts
     params.require(:topic_id)
-    params.permit(:post_ids, :post_number, :username_filters, :filter)
+    params.permit(:post_ids, :post_number, :username_filters, :filter, :include_suggested)
+
+    include_suggested = params[:include_suggested] == "true"
 
     options = {
       filter_post_number: params[:post_number],
       post_ids: params[:post_ids],
       asc: ActiveRecord::Type::Boolean.new.deserialize(params[:asc]),
-      filter: params[:filter]
+      filter: params[:filter],
+      include_suggested: include_suggested,
+      include_related: include_suggested,
     }
 
     fetch_topic_view(options)

lib/topic_view.rb

@@ -51,6 +51,9 @@ class TopicView
     @post_number = [@post_number.to_i, 1].max
     @page = [@page.to_i, 1].max
 
+    @include_suggested = options.fetch(:include_suggested) { true }
+    @include_related = options.fetch(:include_related) { true }
+
     @chunk_size =
       case
       when @print then TopicView.print_chunk_size
@@ -402,11 +405,19 @@ class TopicView
   end
 
   def suggested_topics
-    @suggested_topics ||= TopicQuery.new(@user).list_suggested_for(topic, pm_params: pm_params)
+    if @include_suggested
+      @suggested_topics ||= TopicQuery.new(@user).list_suggested_for(topic, pm_params: pm_params)
+    else
+      nil
+    end
   end
 
   def related_messages
-    @related_messages ||= TopicQuery.new(@user).list_related_for(topic, pm_params: pm_params)
+    if @include_related
+      @related_messages ||= TopicQuery.new(@user).list_related_for(topic, pm_params: pm_params)
+    else
+      nil
+    end
   end
 
   # This is pending a larger refactor, that allows custom orders

spec/requests/topics_controller_spec.rb

@@ -1720,6 +1720,9 @@ RSpec.describe TopicsController do
     let(:topic) { post.topic }
 
     it 'returns first post of the topic' do
+      # we need one for suggested
+      create_post
+
       get "/t/#{topic.id}/posts.json"
 
       expect(response.status).to eq(200)
@@ -1727,6 +1730,13 @@ RSpec.describe TopicsController do
       body = JSON.parse(response.body)
 
       expect(body["post_stream"]["posts"].first["id"]).to eq(post.id)
+
+      expect(body["suggested_topics"]).to eq(nil)
+
+      get "/t/#{topic.id}/posts.json?include_suggested=true"
+      body = JSON.parse(response.body)
+
+      expect(body["suggested_topics"]).not_to eq(nil)
     end
 
     describe 'filtering by post number with filters' do