table = Arel::Table.new(:users)
table.to_sql # => `users`
table[:name].to_sql # => `users`.`name`

# Calculate total money paid.
sql = "SELECT SUM(orders.price * (100 - orders.discount) / 100) as total 
       FROM orders 
       INNER JOIN payments ON payments.order_id = orders.id;"

# Calculate total money paid.
t_order   = Order.arel_table
t_payment = Payment.arel_table
sql       = "SELECT SUM(#{t_order[:price]} * (100 - #{t_order[:discount]}) / 100) as total 
             FROM #{t_order} 
             INNER JOIN #{t_payment} ON #{t_payment[:order_id]} = #{t_order[:id]};"

gem 'arel-to-sql', '~> 1.0'