Nested form not appearing (simple_form) - ruby-on-rails

Question

I have a model named Page and one named Medium. Medium is for uploaded images. The two models are connected with a rich HABTM association.
I want to be able to upload images through my page edit form. However i got a strange result. I am only getting the file field when the page already have an image associated with it. What can cause this?
This is my page model:
class Page < ActiveRecord::Base
attr_accessible :content, :title, :url, :status, :excerpt, :media_attributes
belongs_to :users
has_many :page_medium
has_many :media, :through => :page_medium
accepts_nested_attributes_for :media
acts_as_url :title, :only_when_blank => true, :sync_url => true
acts_as_list
STATUS = %w[draft published]
def to_param
"#{url}" # or whatever you set :url_attribute to
end
validates :title, :status, :url, :presence => true
end
This is my form:
<%= simple_form_for #page, :html => { :class => 'page-form' } do |f| %>
<div class="span9">
<div class="row">
<%= f.input :title, input_html: { class: 'span6'}, wrapper_html: { class: 'span6'} %>
<%= f.input :url, input_html: { class: 'span3'}, wrapper_html: { class: 'span3'} %>
</div>
<div class="tabbable"> <!-- Only required for left/right tabs -->
<ul class="nav nav-tabs">
<li class="active">Content</li>
<li>Excerpt</li>
</ul>
<div class="tab-content">
<div class="tab-pane active" id="tab1">
<%= f.input :content, label: false, input_html: { class: 'wysihtml5 span9' } %>
</div>
<div class="tab-pane" id="tab2">
<%= f.input :excerpt, label: false, input_html: { class: 'wysihtml5 span9 excerpt' }%>
</div>
</div>
</div>
</div>
<div class="span3">
<div class="well">
<fieldset>
<legend>Options</legend>
</fieldset>
<%= f.input :status , collection: Page::STATUS.map { |s| [s.humanize, s] }, prompt: '- Select page status -' %>
<div class="form-actions">
<%= f.button :submit, :class => 'btn btn-primary' %>
<%= link_to t('.cancel', :default => t("helpers.links.cancel")),
pages_path, :class => 'btn' %>
</div>
</div>
</div>
<div class="span9">
<%= f.simple_fields_for :media do |m| %>
<%= m.input :image, as: :file %>
<% end %>
</div>
<% end %>
<%= render :partial => 'display_images' %>
Please comment if you need any more information or code. All tips, answers or constructive comments are appreciated. :)

Answer 1

this code:
<%= f.simple_fields_for :media do |m| %>
<%= m.input :image, as: :file %>
<% end %>
will only allow you to update the images you already have.
To add more images I suggest you follow this episode of railscast: http://railscasts.com/episodes/196-nested-model-form-revised