How do you access Doctrine DBAL in a Symfony2 service class?

I'm learning Symfony2 (and OOP) and want to create a service that's available throughout my app. This service takes a value foo, checks it against a database table, and returns a value bar.

I have a little class

namespace Acme\TestBundle\Toolbox;

class StringToolbox
{
    public function lookupSomething($foo)
   {

        $conn = $this->get('database_connection');
        $sql = "SELECT bar FROM bar_list WHERE foo = :foo";
        $stmt = $conn->prepare($sql);
        $stmt->bindValue("foo", $foo);
        $stmt->execute();


        return $bar;
    }


}

My settings are:

services:
    toolbox:
       class:        Acme\TestBundle\Toolbox
        arguments:   [@database_connection]

But it throws an error saying that the get() method is undefined. I'm stuck-- how can I use DBAL in the service? Thanks!