example.rb

class Remittance < ActiveRecord::Base
  has_many :debits
  
  scope :add_debits_amount, -> { group("remittances.id").joins(:debits).select("remittances.*, sum(debits.amount) AS _debits_amount") }
end

class Debit < ActiveRecord::Base
  belongs_to :remittance
end

Remittance.add_debits_amount.map(&:_debits_amount) #=> [ ... ] Works
Remittance.add_debits_amount.count # => ActiveRecord::StatementInvalid: Mysql2::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '*, sum(debits.amount) AS _debits_amount) AS count_remittances_all_sum_debits_amo' at line 1: SELECT COUNT(remittances.*, sum(debits.amount) AS _debits_amount) AS count_remittances_all_sum_debits_amount_as__debits_amount, remittances.id AS remittances_id FROM `remittances` INNER JOIN `debits` ON `debits`.`remittance_id` = `remittances`.`id` AND `debits`.`deleted_at` IS NULL WHERE `remittances`.`deleted_at` IS NULL GROUP BY remittances.id