2 models in a form in rails 3 - ruby-on-rails-3

Question

I am very new to rails development.
I am creating a simple backend for my portfolio site.
I am not sure about the title of this question. A previous question I asked maybe too convoluted. So I am simplifying it.
Im using 3 models: Post, Attachment, Attachment_Category
I have a form that I use to:
Draft the post with a title, content and a category.
Display attachment categories in a drop down (slideshow, image, video)
Upload the attachment(s).
I have implemented steps 1 and 2.
For step 3: I want it so that when I finally hit submit on the form, the attachment_category_id is saved to the attachment table.
I have the following relationships:
Post.rb
class Post < ActiveRecord::Base
has_many :attachment_categories, :through => :attachments
has_many :attachments,:dependent => :destroy
accepts_nested_attributes_for :attachments
validates_presence_of :title, :content, :category
end
Attachment.rb
class Attachment < ActiveRecord::Base
belongs_to :post
belongs_to :attachment_category
#paperclip
has_attached_file :photo, :styles =>{
:thumb => "100x100#",
:small => "400x400>"
}
end
Attachment_category.rb
class AttachmentCategory < ActiveRecord::Base
has_many :posts , :through => :attachments
has_many :attachments
validates :category_name, :presence =>true
end

Answer 1

So I have accomplished Steps 1, parts of step 2 and step 3.
With my solution, I am able to upload just one attachment.
But it works: The attachment gets saved to the Attachments table with the post_id and the attachment_category_id.
The following code is from _form.html.erb which gets sent to post_controller.rb.
Truncated code:
.....
<%= f.fields_for :attachments do |attach| %> <br>
<%= attach.collection_select :attachment_category_id, AttachmentCategory.all, :id, :category_name %>
<%= attach.file_field :photo %> <br>
<% end %>
.....