title: Reverse engineering a database with tables having no primary keys using Doctrine/Symfony2
body: <p>Am trying to reverse engineer a database using Doctrine this way:</p>
  
  <pre><code>php app/console  doctrine:mapping:convert xml ./src/Acme/TxBundle/Resources/config/doctrine/metadata/orm --from-database --force
  </code></pre>
  
  <p>or this way too (got the same issue):</p>
  
  <pre><code>php app/console doctrine:mapping:import TxBundle
  </code></pre>
  
  <p>But i allways get the below error:</p>
  
  <pre><code>PHP Fatal error:  Call to a member function getColumns() on a non-object in /var/www/Symfony/vendor/doctrine/lib/Doctrine/ORM/Mapping/Driver/DatabaseDriver.php on line 134
  Fatal error: Call to a member function getColumns() on a non-object in /var/www/Symfony/vendor/doctrine/lib/Doctrine/ORM/Mapping/Driver/DatabaseDriver.php on line 134
  </code></pre>
  
  <p>It appears that i have some tables with no primary keys, i need to know if it is possible to make Doctrine work with these tables without altering the table structure to add the needed PKs ?</p>