/**
  * Generates the INSERT SQL used by the persister to persist entities.
  * 
  * @return string
  */
 protected function _generateInsertSql()
 {
     $insertSql = '';
     $columns = $this->_getInsertColumnList();
     if (empty($columns)) {
         $insertSql = $this->_platform->getEmptyIdentityInsertSql($this->_class->getQuotedTableName($this->_platform), $this->_class->getQuotedColumnName($this->_class->identifier[0], $this->_platform));
     } else {
         $columns = array_unique($columns);
         $values = array_fill(0, count($columns), '?');
         $insertSql = 'INSERT INTO ' . $this->_class->getQuotedTableName($this->_platform) . ' (' . implode(', ', $columns) . ') ' . 'VALUES (' . implode(', ', $values) . ')';
     }
     return $insertSql;
 }