title: how to resolve symfony doctrine:build-schema error (Unknown relation alias table_name)
body: <p>how to resolve this symfony error :</p>
  
  <pre><code>C:\inetpub\wwwroot\project\trunk\preprod\signup&gt;php symfony doctrine:build-schema --trace
  &gt;&gt; doctrine  generating yaml schema from database
  
  
    [sfException]
    Unknown relation alias table_name
  
  
  Exception trace:
    at C:\inetpub\wwwroot\project\trunk\preprod\signup\lib\vendor\symfony\lib\plugins\sfDoctrinePlugin\lib\cli\sfDoctrineCli.class.php:69
   sfDoctrineCli-&gt;notifyException at C:\inetpub\wwwroot\ project\trunk\preprod\signup\lib\vendor\symfony\lib\plugins\sfDoctrinePlugin\lib\ven
  dor\doctrine\Doctrine\Cli.php:93
   Doctrine_Cli-&gt;run at C:\inetpub\wwwroot\project\trunk\preprod\signup\lib\vendor\symfony\lib\plugins\sfDoctrinePlugin\lib\task\sfDoctrineB
  aseTask.class.php:112
   sfDoctrineBaseTask-&gt;callDoctrineCli at C:\inetpub\wwwroot\project\trunk\preprod\signup\lib\vendor\symfony\lib\plugins\sfDoctrinePlugin\li
  b\task\sfDoctrineBuildSchemaTask.class.php:57
   sfDoctrineBuildSchemaTask-&gt;execute at C:\inetpub\wwwroot\project\trunk\preprod\signup\lib\vendor\symfony\lib\task\sfBaseTask.class.php:63
  
   sfBaseTask-&gt;doRun at C:\inetpub\wwwroot\project\trunk\preprod\signup\lib\vendor\symfony\lib\task\sfTask.class.php:77
   sfTask-&gt;runFromCLI at C:\inetpub\wwwroot\ project\trunk\preprod\signup\lib\vendor\symfony\lib\command\sfSymfonyCommandApplication.class.ph
  p:76
   sfSymfonyCommandApplication-&gt;run at C:\inetpub\wwwroot\project\trunk\preprod\signup\lib\vendor\symfony\lib\command\cli.php:20
   include at C:\inetpub\wwwroot\project\trunk\preprod\signup\symfony:14
  </code></pre>