Nested Form

nested_form.erb

<%= form_for @playlist do |f| %>
  <%= f.text_field :name, placeholder: 'Name' %>
  <%= f.text_area :description %>
  <%= f.date_field :date_in %>
  <%= f.date_field :date_out %>
  <%= f.fields_for :media_files do |ff| %>
    <%= ff.collection_select :media_file_id, MediaFile.all, :id, :name, {}, {} %>
    <%= ff.select :position, options_for_select((1..50).step(1) {}
    <%= ff.link_to_remove 'Remove from Playlist' %>
  <% end %>
  <%= f.link_to_add 'Add Media File', :media_files %>
<% end %>