public function MarkRead($ConversationID, $ReadingUserID) {
      // Update the the read conversation count for the user.
      $this->SQL->Update('UserConversation uc')
         ->Join('Conversation c', 'c.ConversationID = uc.ConversationID')
         ->Set('uc.CountReadMessages', 'c.CountMessages', FALSE)
         ->Set('uc.DateLastViewed', Gdn_Format::ToDateTime())
         ->Set('uc.LastMessageID', 'c.LastMessageID', FALSE)
         ->Where('c.ConversationID', $ConversationID)
         ->Where('uc.ConversationID', $ConversationID)
         ->Where('uc.UserID', $ReadingUserID)
         ->Put();
         
      // Also update the unread conversation count for this user
      $CountUnread = $this->CountUnread($ReadingUserID);
         
      // Also write through to the current session user.
      if($ReadingUserID > 0 && $ReadingUserID == Gdn::Session()->UserID)
         Gdn::Session()->User->CountUnreadConversations = $CountUnread;
   }