How can I convert this code to raw SQL and use in rails?

How can I convert this code to raw sql and use in rails? Because When I deploy this code in Heroku, there is a request timeout error. I think this will be faster if I use raw sql.

@payments = PaymentDetail.joins(:project).order('payment_details.created_at desc')
@payment_errors = PaymentError.joins(:project).order('payment_errors.created_at desc')

@all_payments = (@payments + @payment_errors)

@all_payments = PaymentDetail.joins(:project).order('payment_details.created_at desc')
  .union(PaymentError.joins(:project).order('payment_errors.created_at desc'))
  .order('created_at desc')

You can always call to_sql to see what the SQL is at the end of every query