title: Multiple primary keys in a schema
body: <p>I have this schema which I need to define two primary keys; one is Drupal's 'vid' field and the other one is my site 'bid' field which is of auto increment type which in turn requires it to be a primary key: wtherwise I get MySQL error. I'm having trouble finding syntax for defining multiple primary keys in a Drupal schema. If anyone can help me out with the syntax, I pretty much appreciate it.</p>
  
  <pre><code>$schema['rft'] = array(
      'fields' =&gt; array(
          'vid' =&gt; array(
              'type' =&gt; 'int',
              'unsigned' =&gt; TRUE,
              'not null' =&gt; TRUE,
              'default' =&gt; 0,
          ),
          'nid' =&gt; array(
              'type' =&gt; 'int',
              'unsigned' =&gt; TRUE,
              'not null' =&gt; TRUE,
              'default' =&gt; 0,
          ),
          'bid' =&gt; array(
              'type' =&gt; 'serial',
              'size' =&gt; 'medium',
              'not null' =&gt; TRUE,                         
          ),
  
      ),
      'indexes' =&gt; array(
          'nid' =&gt; array('nid'),
      ),
      'primary key' =&gt; array('vid'),  //array('vid','bid') doesn't work
  );
  
  return $schema;
  }
  </code></pre>