Doctrine's built-in support for multiple connections follows a baffling use-case that allows different Model objects to be saved on different connections. A more common usage of multiple connections is to send read commands to a slave database and write commands to a master. Here's how to do it.

1. Declare your connections in databases.yml as usual, name one of them master and one of them slave.

2. Override Doctrine's default Query and Record objects. Edit config/ProjectConfiguration.class.php and add the following method:

   public function configureDoctrine(Doctrine_Manager $manager)
   {
       // Configure custom query and custom record classes
       $manager->setAttribute(Doctrine::ATTR_QUERY_CLASS, 'MyQuery');
       $options = array('baseClassName' => 'MyRecord');
       sfConfig::set('doctrine_model_builder_options', $options);  
   }
    

3. Create your MyQuery class in lib/MyQuery

   class MyQuery extends Doctrine_Query
   {
     public function preQuery()
     {
         // If this is a select query then set connection to the slave
         if (Doctrine_Query::SELECT == $this->getType()) {
             $this->_conn = Doctrine_Manager::getInstance()->getConnection('slave');
         // All other queries are writes so they need to go to the master
         } else {
             $this->_conn = Doctrine_Manager::getInstance()->getConnection('master');
         }
     }
   }
    

4. Create your MyRecord class in lib/MyRecord.class.php

   abstract class MyRecord extends sfDoctrineRecord
   {
       public function save(Doctrine_Connection $conn = null)
       {
         $conn = Doctrine_Manager::getInstance()->getConnection('master');
         parent::save($conn);
       }
   }
    

5. If you're using a Doctrine behaviour that creates new tables, like the Versionable behaviour, tell it to use MyRecord too.

   Entity:
     ActAs:
       Versionable:
         builderOptions:
           baseClassName: MyRecord
    

6. Finally, rebuild your model

   ./symfony doctrine:build --all-classes
    

If you have multiple slaves, modify the ProjectConfiguration to pick one on startup then change the method on your Query object to retrieve it.