Question

I have a model, let’s say Cat, and I want to create a new model, Owner. A Cat has_one :owner, but when I created the Cat model, the Owner model didn’t yet exist.

Without resorting to backfilling the new Owner model for each Cat, I want to have an easy way so if I call @cat.owner.something, and @cat.owner doesn’t exist yet, it will call @cat.create_owner on the fly and return it.

I’ve seen this done a few different ways, but I am wondering what is the most Rails-esque way of tackling this, since I need to do this quite often.

Answer 1

I haven’t seen this done before but decided to give it a shot anyways.

I first aliased the association method owner in the Cat model to keep a backup of the original method. I overrode the owner method to call the build_owner method (returns a new Owner object through the association) if the original method returns nil. Otherwise, return the original_owner_method object.

class Cat < ActiveRecord::Base
  has_one :owner

  alias :original_owner_method :owner

  def owner
    if original_owner_method.nil? 
      build_owner
    else
      original_owner_method
    end
  end

So now if you call:
cat = Cat.first

Assuming it doesn’t have an owner, it will build a new Owner object when you call:
cat.owner.name

It will return nil, but still build the owner object on the cat.owner part of the chain without calling method_missing.