a little hack

category.rb

def facets(facets, options = {})
  return {} if facets.blank?

  only_roots = options.key?(:only_roots) ? options.delete(:only_roots) : true # only use roots for facets by default

  facets.inject(ActiveSupport::OrderedHash.new) do |category_facets, (category_ids_string, count)|
    category_ids = category_ids_string.split(',').map(&:to_i)
    category_ids = category_ids[0,1] if only_roots # only use root element if we want only roots

    category_ids.each_with_index do |category_id, i|
      category_facets[category_id] ||= Facet.new(category_id, 0)
      category_facets[category_id].add_count(count)
    end
    category_facets
  end
end

product.rb

define_index do
  indexes :name
  indexes description
  indexes brand_name
  indexes tags.name,     :as => :tag_list
  indexes merchant.name, :as => :merchant_name
  indexes category.path, :as => :category_names # keeping it simple

  has merchant_id, :facet => true
  has active, :type => :boolean
  has "CRC32(`brand_name`)", :as => :brand, :type => :integer, :facet => true
  # using string attributes for filtering is not yet functional in Sphinx 1.10 beta
  # however, we could do it the same way as with categories ...
  # has brand_name, :as => :brand, :type => :string, :facet => true
  has category.path_ints, :as => :category_ids, :type => :multi
  has category.path_ints, :as => :category_path, :type => :string, :facet => true

  set_property :field_weights => {
    :category_names => 10,
    :name           => 5
  }
end

thinking_sphinx.rb

module ThinkingSphinx
  Attribute.class_eval do
    def type_to_config
      {
        :multi    => :sql_attr_multi,
        :datetime => :sql_attr_timestamp,
        # :string   => :sql_attr_str2ordinal,
        :string   => :sql_attr_string, # works for Sphinx >= 1.10 beta
        :float    => :sql_attr_float,
        :boolean  => :sql_attr_bool,
        :integer  => :sql_attr_uint,
        :bigint   => :sql_attr_bigint
      }[type]
    end
  end
end

# We need to avoid category_path facets being translated because this always
# instantiates shitloads of products => slow as hell.
# NOTE: commenting this out changes nothing – the error still happens ...
module ThinkingSphinx
  Facet.class_eval do
    def value(object, attribute_hash)
      attribute_value = attribute_hash['@groupby']

      case column.__name
      when :path_ints
        return attribute_hash['category_path'] if attribute_hash.key?('category_path')
      end

      return translate(object, attribute_value) if translate? || float?

      case @property.type
      when :datetime
        Time.at(attribute_value)
      when :boolean
        attribute_value > 0
      else
        attribute_value
      end
    end
  end

  FacetSearch.class_eval do
    def translate?(name)
      return false if %w(category_path_facet).include?(name)
      facet = facet_from_name(name)
      facet.translate? || facet.float?
    end
  end
end