FEATURE: Show user who posted accepted answer second (#156)

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

plugin.rb

@@ -686,4 +686,54 @@ 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|
+    answer_post_ids = TopicCustomField
+      .select('value::INTEGER')
+      .where(name: 'accepted_answer_post_id')
+      .where(topic_id: topics.map(&:id))
+    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