where do you put helper functions for fixture generation?

I'm writing a fixture in test/fixtures/parrots_model.yml, and I'm
happily using ERB to flesh out my fixture. Suddenly, I realize that I
need a helper function to call from my erb code, such as:

polly:
  subtype: parakeet
  weight_kg: <%= ounces_to_kg(22.3) %>

alex:
  subtype: african gray
  weight_kg: <%= ounces_to_kg(170.3) %>

Where's the right place to stash the "ounces_to_kg()" function so that
it's available when I rake db:load:fixtures? (And does the answer
change if its a method specific to the parrot model?)

tia.