Use specific mysql index with rails

I have this ActiveRecord query

issue = Issue.find(id)
issue.articles.includes(:category).merge(Category.where(permalink: perma))

And the translated to mysql query

SELECT `articles`.`id` AS t0_r0, `articles`.`title` AS t0_r1, 
       `articles`.`hypertitle` AS t0_r2, `articles`.`html` AS t0_r3,
       `articles`.`author` AS t0_r4, `articles`.`published` AS t0_r5,
       `articles`.`category_id` AS t0_r6, `articles`.`issue_id` AS t0_r7,
       `articles`.`date` AS t0_r8, `articles`.`created_at` AS t0_r9, 
       `articles`.`updated_at` AS t0_r10, `articles`.`photo_file_name` AS t0_r11,
       `articles`.`photo_content_type` AS t0_r12, `articles`.`photo_file_size` AS t0_r13,
       `articles`.`photo_updated_at` AS t0_r14, `categories`.`id` AS t1_r0,
       `categories`.`name` AS t1_r1, `categories`.`permalink` AS t1_r2,
       `categories`.`created_at` AS t1_r3, `categories`.`updated_at` AS t1_r4,
       `categories`.`issued` AS t1_r5, `categories`.`order_articles` AS t1_r6 
        FROM `articles` LEFT OUTER JOIN `categories` ON 
       `categories`.`id` = `articles`.`category_id` WHERE 
       `articles`.`issue_id` = 409 AND `categories`.`permalink` = 'Διεθνή' LIMIT 1

In the explation of this query I saw that uses wrong index

+----+-------------+------------+-------+---------------------------------------------------------------------------+-------------------------------+---------+-------+------+----------+-------------+
| id | select_type | table      | type  | possible_keys                                                             | key                           | key_len | ref   | rows | filtered | Extra       |
+----+-------------+------------+-------+---------------------------------------------------------------------------+-------------------------------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | categories | const | PRIMARY,index_categories_