Conditional unique validator and index

Doctrine currently doesn't support conditional unique keys. One commonly happening situation is when we have a model with a column (eg. named "deleted") indicates whether the record was deleted and should no longer be treated as active. If we have an unique index on a "name" column, the index should contain only records with deleted = false. It also concerns an unique validation.

I have prepared a patch for Doctrine_Validator_Unique and Doctrine_Export_Pgsql (Postgresql natively supports conditional indexes).

Example of usage:

public function setTableDefinition()
{
  $this->hasColumn('id', 'integer', 4, array('primary' => true, 'autoincrement' => true, 'sequence' => 'model_id_seq'));
  $this->hasColumn('name', 'string', 128, array('notnull', 'unique' => array('where' => 'deleted = false')));
  $this->hasColumn('deleted', 'boolean', 1, array('notnull', 'default' => false));
  $this->index('model_unique_name', array('fields' => array('name'), 'where' => 'deleted = false', 'type' => 'unique' ));
}