I would like to validate that customer_price >= my_price. I tried the following:

class Product < ActiveRecord::Base
  attr_accessor :my_price
  validates_numericality_of :customer_price, :greater_than_or_equal_to => my_price
  ...
end

(customer_price is a column in the Products table in the database, while my_price isn't.)

Here is the result:

NameError in ProductsController#index
undefined local variable or method `my_price' for #<Class:0x313b648>

What is the right way to do this in Rails 3 ?