Request-scope variable that I can use across all instances of a Rails model class

Question

I'm writing a Rails app and I have a variable whose value takes a significant time to calculate. The variable must be re-calculated at the beginning of each request.

If possible, I'd like to access this variable from different model classes but I'll settle for one calculation per model.

EDIT: e.g. something like this:

module MyHelper
  def get_my_var
    @@my_var ||= some_expensive_calculation
  end
end

So that I can call get_my_var from any controller or model. The only caveat is that I want @@my_var to be recalculated on every request.

Any ideas?