FIX: correctly count participants when more than 24

Also cuts out one query for the normal case

app/serializers/topic_view_serializer.rb

@@ -279,7 +279,7 @@ class TopicViewSerializer < ApplicationSerializer
   end
 
   def participant_count
-    object.participants.size
+    object.participant_count
   end
 
   private

lib/topic_view.rb

@@ -275,9 +275,11 @@ class TopicView
     end
   end
 
+  MAX_PARTICIPANTS = 24
+
   def post_counts_by_user
     @post_counts_by_user ||= begin
-      post_ids = unfiltered_posts.pluck(:id)
+      post_ids = unfiltered_post_ids
 
       return {} if post_ids.blank?
 
@@ -288,13 +290,30 @@ class TopicView
            AND user_id IS NOT NULL
       GROUP BY user_id
       ORDER BY count_all DESC
-         LIMIT 24
+         LIMIT #{MAX_PARTICIPANTS}
       SQL
 
       Hash[Post.exec_sql(sql, post_ids: post_ids).values]
     end
   end
 
+  def participant_count
+    @participant_count ||=
+      begin
+        if participants.size == MAX_PARTICIPANTS
+          sql = <<~SQL
+            SELECT COUNT(DISTINCT user_id)
+            FROM posts
+            WHERE id IN (:post_ids)
+            AND user_id IS NOT NULL
+          SQL
+          Post.exec_sql(sql, post_ids: unfiltered_post_ids).getvalue(0, 0).to_i
+        else
+          participants.size
+        end
+      end
+  end
+
   def participants
     @participants ||= begin
       participants = {}
@@ -354,6 +373,17 @@ class TopicView
     @filtered_post_ids ||= filtered_post_stream.map { |tuple| tuple[0] }
   end
 
+  def unfiltered_post_ids
+    @unfiltered_post_ids ||=
+      begin
+        if @contains_gaps
+          unfiltered_post.pluck(:id)
+        else
+          filtered_post_ids
+        end
+      end
+  end
+
   protected
 
   def read_posts_set