FIX: show private message topic count on admin dashboard reports

app/models/topic.rb

@@ -191,6 +191,8 @@ class Topic < ActiveRecord::Base
     where("topics.category_id IS NULL OR topics.category_id IN (SELECT id FROM categories WHERE #{condition[0]})", condition[1])
   }
 
+  scope :with_subtype, ->(subtype) { where('topics.subtype = ?', subtype) }
+
   attr_accessor :ignore_category_auto_close
   attr_accessor :skip_callbacks
 
@@ -1307,6 +1309,10 @@ SQL
     MiniSuffix.domain(URI.parse(URI.encode(self.featured_link)).hostname)
   end
 
+  def self.private_message_topics_count_per_day(start_date, end_date, topic_subtype)
+    private_messages.with_subtype(topic_subtype).where('topics.created_at >= ? AND topics.created_at <= ?', start_date, end_date).group('date(topics.created_at)').order('date(topics.created_at)').count
+  end
+
   private
 
   def update_category_topic_count_by(num)