title: Validate uniqueness on combined fields in DataMapper
body: <p>I want to be able to put entries in my database where the manufacturer will be represented multiple times but not the same combination of manufacturer and model.
  So "Sony(manufacturer), Tv(model)" is okay "Sony(manufacturer), OtherTv(model)" but the third entry "Sony(manufacturer), Tv(model)" is not okay since the combination of manufacturer and model is not unique. I tried with the <code>:key =&gt; true</code> validation but it doesn't seem to work. And I cannot do something like <code>validates_uniqueness_of :manufacturer AND :model</code> I guess. So how do you do it?</p>
  
  <pre><code>class Tvs
    include DataMapper::Resource
  
    property :id,           Serial
    property :manufacturer, String, :key =&gt; true
    property :model,        String, :key =&gt; true
  
    validates_uniqueness_of :
  end
  </code></pre>