Rails nested model forms: validate presence of foreign key unless user creates new record

Question

In have a pretty simple rails app, here's my code:

class Post < ActiveRecord::Base
  belongs_to :category
  attr_accessible :category_id, :category_attributes    
  accepts_nested_attributes_for :category, :reject_if => :all_blank
end

class Category < ActiveRecord::Base
  has_many :posts
end

#app/views/posts/_form.html.haml
= simple_form_for(@post) do |f|
  = f.association :category
  = f.simple_fields_for :category do |cat_f|
    = cat_f.input :name    

So when creating a new post, I have the option to choose a category (from the select menu) or create a new one (if it doesn't exist).

I want to validate that category_id is present, unless the user opts to create a new category

I'm guessing there's some kind of rails way to solve this problem - I know that I can't just add validates :category_id, :presence => true as this will cause form submission to fail when the user decides to create a new category (and doesn't select one from the drop-down).

Second Question: I recently read a useful rails tutorial that showed you how to toggle between displaying the category select menu and the new category fields so that only one of the two is present on screen at any given time. Anyone got a link to something like that?

Answer 1

I think I have fixed this by replacing:

validates :category_id, :presence => true

with

validates :category, :presence => true

It seems to work. Strange one.

PS

I can only imagine that this works because :category is considered present if the user selects something from the drop-down list OR if they create a new category using the nested form, whereas previously, with my original code, :category_id was only considered present when the user selected something from the drop-down and NOT when they created a new record.