'xf_conversation_master' => array(
'conversation_id' => array('type' => self::TYPE_UINT, 'autoIncrement' => true),
...
'user_id' => array('type' => self::TYPE_UINT, 'required' => true),
'username' => array('type' => self::TYPE_STRING, 'required' => true, 'maxLength' => 50),
...
'last_message_username' => array('type' => self::TYPE_STRING, 'default' => '', 'maxLength' => 50),