#CONTROLLER

# GET /mobiles/archive/
def archive

  @mobiles = Mobile.all
  @mobile_brands = @mobiles.group_by { |m| m.brand }

end

#VIEW

<% @mobile_brands.sort.each do |brand, mobiles| %>
  <h2><%= brand.name %></h2>
  <% for mobile in mobiles %>
    <div class="mobile">
      <strong><%= mobile.name %></strong>
    </div>
  <% end %>
<% end %>

# ERROR

undefined method `<=>' for #<Brand:0x24f6954>

Extracted source (around line #3):

1:
2:
3: <% @mobile_brands.sort.each do |brand, mobiles| %>
4:   <h2>hej</h2>
5:   <% for mobile in mobiles %>
6:     <div class="task">