follow_migrated.rb

def migrated_accounts(account)
  followed = Follow.where(account: account).pluck(:target_account_id)
  migrated = Account.where(id: followed).where.not(moved_to_account_id: nil)
  migrated = migrated.where.not(moved_to_account_id: followed)
  migrated
end

def autosubscribe_migrated_accounts(account_id)
  account = Account.find_by(id: account_id)
  migrated = migrated_accounts(account)
  Account.where(id: migrated.pluck(:moved_to_account_id)).each do |target|
    FollowService.new.call(account, target)
  end
end

autosubscribe_migrated_accounts(1)