FEATURE: Show user who posted accepted answer second (#153)

The user who posts the accepted answer will show as the second poster
in the list of posters from topics list.

config/locales/server.en.yml

@@ -1,4 +1,6 @@
 en:
+  accepted_answer: "Accepted Answer"
+
   site_settings:
     solved_enabled: "Enable solved plugin, allow users to select solutions for topics"
     allow_solved_on_all_topics: "Allow users to select solutions on all topics (by default you control this by editing categories)"

plugin.rb

@@ -686,4 +686,49 @@ SQL
       body: PrettyText.cook(I18n.t('education.topic_is_solved', base_url: Discourse.base_url))
     }
   end
+
+  class ::Topic
+    attr_accessor :accepted_answer_user_id
+  end
+
+  register_topic_list_preload_user_ids do |topics, user_ids, topic_list|
+    Topic.preload_custom_fields(topics, ['accepted_answer_post_id'])
+    answer_post_ids = topics.map { |t| t.custom_fields['accepted_answer_post_id'] }.uniq.compact
+    answer_user_ids = Post.where(id: answer_post_ids).pluck(:topic_id, :user_id).to_h
+    topics.each { |topic| topic.accepted_answer_user_id = answer_user_ids[topic.id] }
+    user_ids.concat(answer_user_ids.values)
+  end
+
+  module AddSolvedToTopicPostersSummary
+    def descriptions_by_id
+      if !defined? @descriptions_by_id
+        super(ids: old_user_ids)
+
+        if id = topic.accepted_answer_user_id
+          @descriptions_by_id[id] ||= []
+          @descriptions_by_id[id] << I18n.t(:accepted_answer)
+        end
+      end
+
+      super
+    end
+
+    def last_poster_is_topic_creator?
+      super || topic.accepted_answer_user_id == topic.last_post_user_id
+    end
+
+    def user_ids
+      if id = topic.accepted_answer_user_id
+        super.insert(1, id)
+      else
+        super
+      end
+    end
+  end
+
+  TopicPostersSummary.class_eval do
+    alias :old_user_ids :user_ids
+
+    prepend AddSolvedToTopicPostersSummary
+  end
 end

spec/requests/list_controller_spec.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe ListController do
+  fab!(:p1) { Fabricate(:post) }
+  fab!(:p2) { Fabricate(:post, topic: p1.topic) }
+  fab!(:p3) { Fabricate(:post, topic: p1.topic) }
+
+  before do
+    SiteSetting.allow_solved_on_all_topics = true
+  end
+
+  it 'shows the user who posted the accepted answer second' do
+    TopicFeaturedUsers.ensure_consistency!
+    DiscourseSolved.accept_answer!(p3, p1.user, topic: p1.topic)
+
+    get '/latest.json'
+    posters = response.parsed_body["topic_list"]["topics"].first["posters"]
+    expect(posters[0]["user_id"]).to eq(p1.user_id)
+    expect(posters[1]["user_id"]).to eq(p3.user_id)
+    expect(posters[1]["description"]).to include("Accepted Answer")
+    expect(posters[2]["user_id"]).to eq(p2.user_id)
+  end
+end