Alarm Grid, a home security store, shows you how to set canonical urls in Spree.

_canonical_url.html.erb

- if @canonical_url
  = tag('link', :rel => 'canonical', :href => "http://" + Spree::Config[:site_url] + URI.parse(@canonical_url).path)
  

Canonical URLS.md

Alarm Grid will show you how to create canonical urls in Spree Commerce. This is only an example for products show. You must implement EVERYWHERE.

canonical_url_override.rb

Deface::Override.new(:virtual_path => 'spree/layouts/spree_application',
                     :name => 'canonical_url',
                     :insert_bottom => "[data-hook='inside_head']",
                     :partial => 'shared/canonical_url')

products_controller_decorator.rb

module Spree
  ProductsController.class_eval do
    before_filter :show_filter, :only => :show

    def show_filter
      @canonical_url = spree.product_path(:id => @product.permalink)
    end
  end
end

Spree 2.0.4 now includes https://github.com/jumph4x/canonical-rails by default