gistfile1.rb

def counter(name)
  name = name.to_s
  @counters ||= {}
  return unless name.present?
  return @counters[name] if counters.key?(name)
  return unless block_given?
  relaion = yield
  @counters[name.to_s] ||=  relation.count if relation.respond_to? :count
end

# использование
= counter('product') { Product }
= counter('uniq_product') { Product.uniq }

= counter('product') - counter('uniq_product')