title: Grails domain class: unique constraint for multiple columns
body: <p>Suppose a simple Grails domain class:</p>
  
  <pre><code>class Account {
      String countryId;
  
      String userName;
  
      String password;
  
      static constraints = {
          ...???...
      }
  }
  </code></pre>
  
  <p>It is required that user names are unique for a particular <i>countryId</i>, thus there must be a <i>unique</i> contraint on two columns. How to express this in the <i>constraints</i> definition? </p>