the query selects all ids from the full branch from the nested set

product.rb

class Product
  define_index do
    # ...
    has %(
      (
        SELECT GROUP_CONCAT(IFNULL(parent.`id`, '0') SEPARATOR ',')
        FROM `categories` AS node,
        `categories` AS parent
        WHERE node.`merchant_id` = `products`.`merchant_id` AND
          parent.`merchant_id` = `products`.`merchant_id` AND
          node.`lft` BETWEEN parent.`lft` AND parent.`rgt` AND node.`id` = `category_id`
        ORDER BY parent.`lft`
      )
    ), :source => :query, :type => :multi, :as => :category_ids, :facet => true
  end
end