Fix article board group N+1 query problem

[injoonH]

Join boardgroup table when selecting articles:

SELECT ...
FROM `core_article`
  ...
    INNER JOIN `core_board`
      ON (`core_article`.`parent_board_id` = `core_board`.`id`)
    LEFT OUTER JOIN `core_boardgroup`
      ON (`core_board`.`group_id` = `core_boardgroup`.`id`)
...