//when adding this
//[...]
  indexes:
    unique_idx:
      fields: externalUserId
      type: unique

//...and calling "php.exe doctrine create tables" i get the following error:

Catchable fatal error: Argument 1 passed to Doctrine_Export_Sqlite::getIndexFieldDeclarationList() must be an array, string given, called in S:\worksp
aces\privat\myproject\lib\Doctrine\Export\Sqlite.php on line 122 and defined in S:\workspaces\privat\myproject\lib\Doctrine\Export\Sqlite.php on line 134