FIX: Select+below will ask server for post ids on megatopics.

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

@@ -152,6 +152,35 @@ export default Ember.Controller.extend(BufferedContent, {
     this.appEvents.trigger("post-stream:refresh", { force: true });
   },
 
+  _updateSelectedPostIds(postIds) {
+    this.get("selectedPostIds").pushObjects(postIds);
+    this.set("selectedPostIds", [...new Set(this.get("selectedPostIds"))]);
+    this._forceRefreshPostStream();
+  },
+
+  _loadPostIds(post) {
+    if (this.get("loadingPostIds")) return;
+
+    const postStream = this.get("model.postStream");
+    const url = `/t/${this.get("model.id")}/post_ids.json`;
+
+    this.set("loadingPostIds", true);
+
+    return ajax(url, {
+      data: _.merge(
+        { post_number: post.get("post_number") },
+        postStream.get("streamFilters")
+      )
+    })
+      .then(result => {
+        result.post_ids.pushObject(post.get("id"));
+        this._updateSelectedPostIds(result.post_ids);
+      })
+      .finally(() => {
+        this.set("loadingPostIds", false);
+      });
+  },
+
   actions: {
     showPostFlags(post) {
       return this.send("showFlags", post);
@@ -627,10 +656,13 @@ export default Ember.Controller.extend(BufferedContent, {
     },
 
     selectBelow(post) {
-      const stream = [...this.get("model.postStream.stream")];
-      const below = stream.slice(stream.indexOf(post.id));
-      this.get("selectedPostIds").pushObjects(below);
-      this._forceRefreshPostStream();
+      if (this.get("model.postStream.isMegaTopic")) {
+        this._loadPostIds(post);
+      } else {
+        const stream = [...this.get("model.postStream.stream")];
+        const below = stream.slice(stream.indexOf(post.id));
+        this._updateSelectedPostIds(below);
+      }
     },
 
     deleteSelected() {

app/controllers/topics_controller.rb

@@ -175,22 +175,34 @@ class TopicsController < ApplicationController
     render_json_dump(wordpress_serializer)
   end
 
+  def post_ids
+    params.require(:topic_id)
+    params.permit(:post_number, :username_filters, :filter)
+
+    options = {
+      filter_post_number: params[:post_number],
+      filter: params[:filter],
+      skip_limit: true,
+      asc: true,
+      skip_custom_fields: true
+    }
+
+    fetch_topic_view(options)
+    render_json_dump(post_ids: @topic_view.posts.pluck(:id))
+  end
+
   def posts
     params.require(:topic_id)
     params.permit(:post_ids, :post_number, :username_filters, :filter)
 
-    default_options = {
+    options = {
       filter_post_number: params[:post_number],
       post_ids: params[:post_ids],
       asc: ActiveRecord::Type::Boolean.new.deserialize(params[:asc]),
       filter: params[:filter]
     }
 
-    if (username_filters = params[:username_filters]).present?
-      default_options[:username_filters] = username_filters.split(',')
-    end
-
-    @topic_view = TopicView.new(params[:topic_id], current_user, default_options)
+    fetch_topic_view(options)
 
     render_json_dump(TopicViewPostsSerializer.new(@topic_view,
       scope: guardian,
@@ -714,6 +726,17 @@ class TopicsController < ApplicationController
     )
   end
 
+  def fetch_topic_view(options)
+    check_username_filters { |usernames| options[:username_filters] = usernames }
+    @topic_view = TopicView.new(params[:topic_id], current_user, options)
+  end
+
+  def check_username_filters
+    if (username_filters = params[:username_filters]).present?
+      yield(username_filters.split(',')) if block_given?
+    end
+  end
+
   def toggle_mute
     @topic = Topic.find_by(id: params[:topic_id].to_i)
     guardian.ensure_can_see!(@topic)

config/routes.rb

@@ -680,6 +680,7 @@ Discourse::Application.routes.draw do
   get "t/:slug/:topic_id/:post_number" => "topics#show", constraints: { topic_id: /\d+/, post_number: /\d+/ }
   get "t/:slug/:topic_id/last" => "topics#show", post_number: 99999999, constraints: { topic_id: /\d+/ }
   get "t/:topic_id/posts" => "topics#posts", constraints: { topic_id: /\d+/ }, format: :json
+  get "t/:topic_id/post_ids" => "topics#post_ids", constraints: { topic_id: /\d+/ }, format: :json
   get "t/:topic_id/excerpts" => "topics#excerpts", constraints: { topic_id: /\d+/ }, format: :json
   post "t/:topic_id/timings" => "topics#timings", constraints: { topic_id: /\d+/ }
   post "t/:topic_id/invite" => "topics#invite", constraints: { topic_id: /\d+/ }

lib/topic_view.rb

@@ -71,7 +71,7 @@ class TopicView
 
     filter_posts(options)
 
-    if @posts
+    if @posts && !@skip_custom_fields
       if (added_fields = User.whitelisted_user_custom_fields(@guardian)).present?
         @user_custom_fields = User.custom_fields_for_ids(@posts.pluck(:user_id), added_fields)
       end
@@ -534,7 +534,7 @@ class TopicView
           .order(sort_order: :desc)
       end
 
-    posts = posts.limit(@limit)
+    posts = posts.limit(@limit) if !@skip_limit
     filter_posts_by_ids(posts.pluck(:id))
 
     @posts = @posts.unscope(:order).order(sort_order: :desc) if !asc

spec/requests/topics_controller_spec.rb

@@ -1451,6 +1451,70 @@ RSpec.describe TopicsController do
     end
   end
 
+  describe '#post_ids' do
+    let(:post) { Fabricate(:post) }
+    let(:topic) { post.topic }
+
+    before do
+      TopicView.stubs(:chunk_size).returns(1)
+    end
+
+    it 'returns the right post ids' do
+      post2 = Fabricate(:post, topic: topic)
+      post3 = Fabricate(:post, topic: topic)
+
+      get "/t/#{topic.id}/post_ids.json", params: {
+        post_number: post.post_number
+      }
+
+      expect(response.status).to eq(200)
+
+      body = JSON.parse(response.body)
+
+      expect(body["post_ids"]).to eq([post2.id, post3.id])
+    end
+
+    describe 'filtering by post number with filters' do
+      describe 'username filters' do
+        let(:user) { Fabricate(:user) }
+        let(:post) { Fabricate(:post, user: user) }
+        let!(:post2) { Fabricate(:post, topic: topic, user: user) }
+        let!(:post3) { Fabricate(:post, topic: topic) }
+
+        it 'should return the right posts' do
+          get "/t/#{topic.id}/post_ids.json", params: {
+            post_number: post.post_number,
+            username_filters: post2.user.username
+          }
+
+          expect(response.status).to eq(200)
+
+          body = JSON.parse(response.body)
+
+          expect(body["post_ids"]).to eq([post2.id])
+        end
+      end
+
+      describe 'summary filter' do
+        let!(:post2) { Fabricate(:post, topic: topic, percent_rank: 0.2) }
+        let!(:post3) { Fabricate(:post, topic: topic) }
+
+        it 'should return the right posts' do
+          get "/t/#{topic.id}/post_ids.json", params: {
+            post_number: post.post_number,
+            filter: 'summary'
+          }
+
+          expect(response.status).to eq(200)
+
+          body = JSON.parse(response.body)
+
+          expect(body["post_ids"]).to eq([post2.id])
+        end
+      end
+    end
+  end
+
   describe '#posts' do
     let(:post) { Fabricate(:post) }
     let(:topic) { post.topic }

test/javascripts/acceptance/topic-test.js.es6

@@ -264,3 +264,26 @@ QUnit.test("selecting posts", async assert => {
     "it should hide the multi select menu"
   );
 });
+
+QUnit.test("select below", async assert => {
+  await visit("/t/internationalization-localization/280");
+  await click(".toggle-admin-menu");
+  await click(".topic-admin-multi-select .btn");
+  await click("#post_3 .select-below");
+
+  assert.ok(
+    find(".selected-posts")
+      .html()
+      .includes(I18n.t("topic.multi_select.description", { count: 18 })),
+    "it should select the right number of posts"
+  );
+
+  await click("#post_2 .select-below");
+
+  assert.ok(
+    find(".selected-posts")
+      .html()
+      .includes(I18n.t("topic.multi_select.description", { count: 19 })),
+    "it should select the right number of posts"
+  );
+});