gistfile1.rb

module TypeColumn
  def type_column(column_name, types = nil)
    types ||= const_get(column_name.to_s.pluralize.upcase)
    raise if types.blank?

    validates_inclusion_of column_name, :in => types
    types.each do |t|
      scope t, where(column_name => t)
      class_eval(%[def #{t}?; #{column_name} == #{t.dump}; end], __FILE__, __LINE__)
    end
  end
end

class Note < ActiveRecord::Base
  extend TypeColumn
  CATEGORIES = %w[foo bar piyo]

  type_column :category
end

Note.foo.all?{|n| n.foo? } # => true