gistfile1.rb

<% @models.each do |model| %>
      <% if model.image_models.first %>
        <% unless model.image_models.first.document_file_name.blank? %>
           <%= image_tag model.image_models.first.permalink %>
        <% end %>
      <% end %>
<% end %>

#Cambiar en el model.rb
has_many :image_models, :as => :owner, :dependent => :destroy, :order => 'created_at DESC'

#por

has_many :image_models, :as => :owner, :dependent => :destroy, :order => 'created_at ASC'