app/views/categories/index.html.erb
<section class="content-header">
</section>
<section class="content">
<div class="box">
<div class="box-header">
<h3 class="box-title">Category</h3>
<div class="box-tools pull-right">
<div class="btn-group">
<%= link_to "Add Category", new_category_path, class: "btn btn-primary" %>
</div>
</div>
</div>
<div class="box-body">
<div class="">
<ul>
<% @categories.each do |category| %>
<li>
<%= category.name %>
<%= link_to '+', new_category_subcategory_path(category_id: category.id) %>
<%= render 'subcategories/subcategories', category: category %>
</li>
<% end %>
</ul>
</div>
</div>
</div>
</section>