Rails sorting based on 2 fields : I have a product model with 'price' field and 'sale_price' field. I want to sort the records according to price in ascending order. But if a product has sale_price value then it should be sorted according to sale_price not by price.
E.g

    Product.id:1, Product.name:watch, Product.price:100, Product.sale_price:nil
    Product.id:2, Product.name:Bag, Product.price:200, Product.sale_price:50
    Product.id:3, Product.name:Shoes, Product.price:300, Product.sale_price:nil

I want this to be sorted as 
1.Bag
2.watch
3.Shoes

Thanks