title: Simple_form, edit multiple associated items
body: <p>I'm trying to edit multiple association in a single form. I'm using simple form and my association is easy:</p>
  
  <pre><code>Class Gallery
   has_many :pictures
  end
  
  Class Pictures
  end
  </code></pre>
  
  <p>Every picture has an image and the caption, but I have already uploaded all the pictures in a separated form: I need now to add the caption text for every picture in a single form.</p>
  
  <p>I try with simple_form doing this:</p>
  
  <pre><code> = simple_form_for [:admin, gallery] do |form|
      - if gallery.pictures.present?
          - gallery.pictures.each do |p|     
              = image_tag(p.image.url(:thumb), height: '50')
  </code></pre>
  
  <p>but I cannot find a way to add the caption text field for every picture of my gallery.</p>
  
  <p>Any hint?</p>