deletable.rb

# config/initializers/deletable.rb
ActiveRecord::Base.class_eval do
  before_destroy :check_deletable

  def self.undeletable_if_used(by_hash)
    unless by_hash[:by]
      raise "Expected hash to have key :by and a value of one or more symbols"
    end
    define_method(:deletable?) do
      ![by_hash[:by]].flatten.detect { |a|
        if self.respond_to? "#{a.to_s}_count"
          self.send("#{a.to_s}_count") > 0
        else
          self.send(a.to_s).count
        end
      }
    end

    (class << self; self; end).send(:define_method, :checks_for_use_by) do 
        [by_hash[:by]].flatten
    end
  end

  # all AR classes now call check_deletable; 
  # don't break if deletable? isn't defined
  def check_deletable
    self.respond_to? "deletable?" ? deletable? : true
  end
end