app/models/metadatum.rb

class Metadatum < ActiveRecord::Base

  belongs_to :metadatable,
    :polymorphic   => true,
    :counter_cache => true

  belongs_to :user

  attr_accessible :key, :value

  validates_format_of :key,
    :with => URI::regexp(Settings.allowed_iri_schemes),
    :if => :key?

end