title: Validation UNIQUE field in Codeigniter with 2 index
body: <p>In the Codeigniter Framework, I can validate an Unique field in the MYSQL Database using the "Form Validation Class". Exemple: </p>
  
  <pre><code>$this-&gt;form_validation-&gt;set_rules('form_field', 'form_label', 'is_unique[table.field]');
  </code></pre>
  
  <p>Work perfectly, but, I need validate a field from a table with 2 index. Exemple:</p>
  
  <pre><code>UNIQUE INDEX `id_aluno` (`id_aluno`, `ano`),
  </code></pre>
  
  <p>The Codeigniter Framework can do it natively?</p>