title: Rails: validate uniqueness of two columns (together)
body: <p>I have a <code>Release</code> model with <code>medium</code> and <code>country</code> columns (among others). There should not be <code>releases</code> that share identical <code>medium</code>/<code>country</code> combinations.</p>
  
  <p>How would I write this as a rails validation? </p>