snippet.html

<h1>New post</h1>
<% form_for(@post) do |f| %>
  <%= f.error_messages %>

  <p>
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </p>
  <p>
    <%= f.label :content %><br />
    <%= f.text_area :content %>
  </p>
	
 <!-- :photos is a model that has paperclip on it. 
  I can use :image with the photomodel and it's action, but when trying to do a nested form
  everything in Post.photos is nil except the timestamp -->
	
 <% f.fields_for @photos, :html => { :multipart => true } do |picture| %>  
  <%= picture.label :image %>
  <%= picture.file_field :image %>
		<% end %> 
  <p>
    <%= f.submit 'Create' %>
  </p>
<% end %>

<%= link_to 'Back', posts_path %>

snippet.rb

class Post < ActiveRecord::Base
  require 'RedCloth'  
  has_many :photos
  
  accepts_nested_attributes_for :photos, :allow_destroy => :true
  
class PhotoPicker < Liquid::Tag                                            
    def initialize(tag_name, photo_and_size, tokens)
       super
       @photo, @size = photo_and_size.split(", ")
       #@size = photo_and_size[1].to_sym
    end

    def render(context)
      context['post'][@photo.to_i].image.url(@size.to_sym)
    end
end 

  Liquid::Template.register_tag('picture', PhotoPicker) 
end