animals_controller.rb

class AnimalsController < ApplicationController
  inherit_resources
  
  ..
  
  protected
  
  def build_resource_params
    animal_params = params.fetch(:animal, {}).permit(:name, :age, :family)
    [prepare_animal_params(animal_params)]
  end
  
  def prepare_animal_params animal_params
    # here we manipulate the model parameters however we want
    # e.g. animal_params[:age] = 1 if animal_params[:age] < 0 
    ...
    # importantly this method must return the model params
    animal_params
  end