class Customers_model extends CI_Model
{
    public function __construct()
    {
        parent::__construct();

        $this->db = $this->load->database($this->user->getDatabase(), TRUE);
    }
}

public function getDatabase()
{
    $config['hostname'] = 'localhost';
    $config['username'] = 'root';
    $config['password'] = '';
    $config['database'] = $this->session->user['database'];
    $config['dbdriver'] = 'mysqli';

    return $config;
}

return array(

    'default' => 'mysql',

    'connections' => array(

        # Primary/Default database connection
        'mysql' => array(
            'driver'    => 'mysql',
            'host'      => '127.0.0.1',
            'database'  => 'database1',
            'username'  => 'root',
            'password'  => 'secret'
            'charset'   => 'utf8',
            'collation' => 'utf8_unicode_ci',
            'prefix'    => '',
        ),

        # Secondary database connection
        'mysql2' => array(
            'driver'    => 'mysql',
            'host'      => '127.0.0.1',
            'database'  => 'database2',
            'username'  => 'root',
            'password'  => 'secret'
            'charset'   => 'utf8',
            'collation' => 'utf8_unicode_ci',
            'prefix'    => '',
        ),
    ),
);