Don’t over-engineer your models in Rails.
You already get what you’re looking for automatically.
Given this class (entire contents)
class Cat < ActiveRecord::Base end
You can do the following:
@cat = Cat.find(1) Will throw exception if you don’t pass in a valid id
@cat = Cat.find_by_id(1) returns nil
There’s no need for your own special API. This is why ActiveRecord exists - to keep you from having to write these methods yourself. #find and its variants are all public class methods. No need to wrap them 
Hope that helps.
