gistfile1.php

<?php

class ConnectionListener extends Doctrine_Connection
  implements Doctrine_EventListener_Interface
{
  protected
    $master = null,
    $slave  = null;

  public function __construct(PDO $master, PDO $slave)
  {
    $this->master = $master;
    $this->slave  = $slave;
  }

  public function preQuery(Doctrine_Event $event)
  {
    $this->forceDbh($event->getInvoker(), 'slave');
  }

  public function postQuery(Doctrine_Event $event)
  {
    $this->restoreDbh($event->getInvoker());
  }

  public function prePrepare(Doctrine_Event $event)
  {
    $use = 0 === strpos(trim(strtolower($event->getQuery())), 'select') ?
      'slave' : 'master';
    $this->forceDbh($event->getInvoker(), $use);
  }

  public function postStmtExecute(Doctrine_Event $event)
  {
    $this->restoreDbh($event->getInvoker()->getConnection());
  }

  public function preExec(Doctrine_Event $event)
  {
    $this->forceDbh($event->getInvoker(), 'master');
  }

  public function postExec(Doctrine_Event $event)
  {
    $this->restoreDbh($event->getInvoker());
  }

  // protected

  protected function forceDbh($conn, $type)
  {
    if ($this->$type !== $conn->dbh)
    {
      $conn->options['previous_dbh'] = $conn->dbh;
      $conn->dbh = $this->$type;
    }
  }

  protected function restoreDbh($conn)
  {
    if (isset($conn->options['previous_dbh']))
    {
      $conn->dbh = $conn->options['previous_dbh'];
      unset($conn->options['previous_dbh']);
    }
  }

  // the remaining methods required by Doctrine_EventListener_Interface

  public function preTransactionCommit(Doctrine_Event $event) { }
  public function postTransactionCommit(Doctrine_Event $event) { }
  public function preTransactionRollback(Doctrine_Event $event) { }
  public function postTransactionRollback(Doctrine_Event $event) { }
  public function preTransactionBegin(Doctrine_Event $event) { }
  public function postTransactionBegin(Doctrine_Event $event) { }
  public function postConnect(Doctrine_Event $event) { }
  public function preConnect(Doctrine_Event $event) { }
  public function postPrepare(Doctrine_Event $event) { }
  public function preStmtExecute(Doctrine_Event $event) { }
  public function preError(Doctrine_Event $event) { }
  public function postError(Doctrine_Event $event) { }
  public function preFetch(Doctrine_Event $event) { }
  public function postFetch(Doctrine_Event $event) { }
  public function preFetchAll(Doctrine_Event $event) { }
  public function postFetchAll(Doctrine_Event $event) { }
}