How do I get my Collection to use a different DB connection?

Question

So I have extended a Magento collection to obtain my own collection only I want to use a different DB connection. What I have attempted to do is define the connection in config.xml like so:

<global>
    <resources>
        <externaldb_write>
            <connection>
                <use>externaldb_database</use>
            </connection>
        </externaldb_write>
        <externaldb_read>
            <connection>
                <use>externaldb_database</use>
            </connection>
        </externaldb_read>
        <externaldb_setup>
            <connection>
                <use>core_setup</use>
            </connection>
        </externaldb_setup>
        <externaldb_database>
            <connection>
                <host><![CDATA[localhost]]></host>
                <username><![CDATA[db_username]]></username>
                <password><![CDATA[db_password]]></password>
                <dbname><![CDATA[db_name]]></dbname>
                <model>mysql4</model>
                <type>pdo_mysql</type>
                <active>1</active>
            </connection>
        </externaldb_database>
    </resources>
</global>

and then in my overriding constructor, do:

class VMR_Customreports_Model_Resource_Storeoverview_Collection extends     Mage_Reports_Model_Resource_Order_Collection
{
        public function __construct()
        {
                parent::__construct();

                $resource = Mage::getSingleton('core/resource');
                $conn = $resource->getConnection('externaldb_read');

                $this->setConnection($conn);
        }
.....
}

This doesn't appear to have any effect. I would have expected an error if it didn't work. What do I do?

UPDATE: After more debugging, I note that this code below returns the original db connection and not the one I defined in my XML. Why is that?

$conn = $resource->getConnection('externaldb_read');

Answer 1

A quick check (see test case below) actually shows what you are doing should be just peachy.
For that reason, I suspect some error in your configuration. Please edit your post to include your collection class with at least it's __construct and _construct methods.
Also, if you are not using the sales/order resource model, please include it with it's constructor methods, too.

UPDATE:
I just noticed you are using $resource->getConnection('reportingdb_read'); in your constructor.
In the configuration XML the connection is called <externaldb_read>.
Note: reportingdb_read != externaldb_read

Maybe thats just a copy & paste error in your post, but if not, you have to use
$resource->getConnection('externaldb_read');
If that isn't the issue, please update your post, again, and I'll help dig further.

---

// Test to confirm a connection set via setConnection() on a 
// Mage_Reports_Model_Resource_Order_Collection instance is used
// to load the collection data.
class Reports_CollectionTest extends PHPUnit_Framework_TestCase
{
    public static function setUpBeforeClass()
    {
        require_once 'app/Mage.php';
        Mage::setIsDeveloperMode(true);
        Mage::app('admin');
    }

    protected function getConnectionMock()
    {
        $mockSelect = $this->getMockBuilder('Varien_Db_Select')
            ->disableOriginalConstructor()
            ->getMock();

        $mockConn = $this->getMockBuilder('Varien_Db_Adapter_Pdo_Mysql')
            ->disableOriginalConstructor()
            ->getMock();

        $mockConn->expects($this->once())
            ->method('select')
            ->will($this->returnValue($mockSelect));

        $mockConn->expects($this->once())
            ->method('fetchAll');

        return $mockConn;
    }

    /**
     * @test
     */
    public function testCollectionConnection()
    {
        $mockCon = $this->getConnectionMock();

        $collection = new Mage_Reports_Model_Resource_Order_Collection;
        $collection->setConnection($mockCon);

        $this->assertSame($mockCon, $collection->getConnection());
        $collection->load();
    }
}