Spree Decorator to add a custom ordering scope. Placed in "app/models/spree/product/"

product_scopes_decorator.rb

Spree::Product.class_eval do
  def self.simple_scopes
    [
      :ascend_by_random,
      :ascend_by_updated_at,
      :descend_by_updated_at,
      :ascend_by_name,
      :descend_by_name,
      # Need to have master price scopes here
      # This makes them appear in admin/product_groups/edit
      :ascend_by_master_price,
      :descend_by_master_price,
      # :descend_by_popularity
    ]
  end

  simple_scopes.each do |name|
    # We should not define price scopes here, as they require something slightly different
    next if name.to_s.include?("master_price")
    next if name.to_s.include?("random")
    parts = name.to_s.match(/(.*)_by_(.*)/)
    order_text = "#{Spree::Product.quoted_table_name}.#{parts[2]} #{parts[1] == 'ascend' ?  "ASC" : "DESC"}"
    self.scope(name.to_s, relation.order(order_text))
  end

  def self.ascend_by_random
    order("RANDOM()")
  end
end

The scope has to be something callable.

So, your line 23, should be:

self.scope name.to_s, -> { relation.order(order_text) }