How can I incorporate this polymorphic model into a nested form, that also creates has_many through associations?

Question

I have a Post model, which has many Post_Images (polymorphic as :imageable). I am trying to make a nested form where Posts can be created, including Post_Images, all while simultaneously assigning the post to groups (there is a group model which has_many posts :through :assignments, hence the form_for [@user, @post] below).

Models:

Post Model

class Post < ActiveRecord::Base
  #   attr_accessible :name, :borrow, :price, :description, :user_id, :product_category_id, :post_image_ids, :group_ids, :post_images_attributes, :post_image
  has_many :post_images, :as => :imageable, :dependent => :destroy
  accepts_nested_attributes_for :post_images, :reject_if => lambda { |t| t[:post_image].nil?}, :allow_destroy => true
.
.
.
end

PostImage model (should be just Image, but I made it ahile ago and can't refactor it):

class PostImage < ActiveRecord::Base
  # attr_accessible :imageable_id, :imageable_type, :post_image_ids
  belongs_to :imageable, :polymorphic => true
  has_attached_file :image, :styles => { :small => "150x150>", :large => "320x240>" }
  validates_attachment_presence :image
  validates_attachment_size :image, :less_than => 5.megabytes

end

PostsController

def new
    @user = User.find(params[:user_id])
    @post = @user.posts.build
    @assigment = Assignment.new
    #allow up to 5 images to be uploaded
    #5.times {@post.build_post_image}
    post_image = @post.post_images.build
    respond_to do |format|
      format.html # new.html.erb
      format.xml  { render :xml => @post }
    end
  end
  def create
    @user = User.find(params[:user_id])
    params[:post][:group_ids] ||= []
    @post = @user.posts.build(params[:post])

    if @post.save
      redirect_to @user
    else
      redirect_to new_user_post_path(current_user)
    end
  end

The blasted form: (builds posts, while also adding them to groups)

=nested_form_for( [@user, @post], :remote => true) do |f|
    =hidden_field_tag 'user_id', @user.id
    =f.fields_for :post_images
    =f.link_to_add "Add an image", :post_images
    =f.label :name
    =f.text_field :name
    =f.label :borrow, "borrow?"
    =f.check_box :borrow
    =f.label :price
    =f.text_field :price        
    =f.label :description
    =f.text_area :description
    %h2 choose which groups you want to post this item to    
    -@user.groups_as_owner.each do |group|
        = label_tag "group_checkbox_#{group.id}", group.name
        = check_box_tag("post[group_ids]", group.id, @user.groups_as_owner.include?(group), :id => "group_checkbox_#{group.id}")
    -@user.groups_as_member.each do |group|
        = label_tag "group_checkbox_#{group.id}", group.name
        = check_box_tag("post[group_ids]", group.id, @user.groups_as_member.include?(group), :id => "group_checkbox_#{group.id}")
    .action=f.submit "Save Post"

_post_image_fields partial

=f.label :post_image, "Image"
%br
=f.file_field :post_image
=f.link_to_remove "remove"

I have attr_accessible commented out because I was getting a WARNING: Can't mass-assign protected attributes and in searching for a resolutio to this, I happened on a bunch of resources including:

nested form with polymorphic models

google group convo addressing mass-assignment warning

railscasts nested form 2

To my knowledge, none of them really addressed my issue. Maybe it is because of the extra aspect I have in my form- the creation of group assignments? It seems to me though that it is a param issue. Because I got it to the point where the form would work...with a lot of attr_accessible in place, but it would say that it couldn't mass-assign to :post_image which I made accessible in PostImages and Posts...so I wasn't sure what the issue with that was.

lastly, is the answer in this post?(i don't quite understand it): ActiveRecord, has_many :through, and Polymorphic Associations

Let me know if I should supply more info, somehow make this clearer. Happy Holidays all!