Sort order on custom column from customer grid

On my Admin side customer grid, i have a situacion, some times and some customers on column name i need First Name + Last Name, it's already done by magento core, but some customer i need just to print First name. Take a look on my Grid.php, i need take off last name from 'name' catenation, but when i remove from collection my column sort stop work.

<?php

class Mage_Adminhtml_Block_Customer_Grid extends Mage_Adminhtml_Block_Widget_Grid
{

public function __construct()
{
    parent::__construct();
    $this->setId('customerGrid');
    $this->setUseAjax(true);
    $this->setDefaultSort('entity_id');
    $this->setSaveParametersInSession(true);
}

protected function _prepareCollection()
{
    $collection = Mage::getResourceModel('customer/customer_collection')
        ->addNameToSelect()
        ->addAttributeToSelect('email')
        ->addAttributeToSelect('created_at')
        ->addAttributeToSelect('group_id')
        ->joinAttribute('billing_postcode', 'customer_address/postcode', 'default_billing', null, 'left')
        ->joinAttribute('billing_city', 'customer_address/city', 'default_billing', null, 'left')
        ->joinAttribute('billing_telephone', 'customer_address/telephone', 'default_billing', null, 'left')
        ->joinAttribute('billing_region', 'customer_address/region', 'default_billing', null, 'left')
        ->joinAttribute('billing_country_id', 'customer_address/country_id', 'default_billing', null, 'left');

    /*foreach($collection as $customer){
        preg_match("/[A-z| ]*//*", $customer->getName(), $name);
        $customer->setName($name[0]);
        unset($name);
    }*/

    $this->setCollection($collection);

    return parent::_prepareCollection();
}

protected function _prepareColumns()
{
    $this->addColumn('entity_id', array(
        'header'    => Mage::helper('customer')->__('ID'),
        'width'     => '50px',
        'index'     => 'entity_id',
        'type'  => 'number',
    ));

    /*
    $this->addColumn('firstname', array(
        'header'    => Mage::helper('customer')->__('First Name'),
        'index'     => 'firstname'
    ));
    $this->addColumn('lastname', array(
        'header'    => Mage::helper('customer')->__('Last Name'),
        'index'     => 'lastname'
    ));
    */

    $this->addColumn('name', array(
        'header'    => Mage::helper('customer')->__('Name'),
        'sortable'  => true,
        'index'     => 'name'
    ));

    $this->addColumn('email', array(
        'header'    => Mage::helper('customer')->__('Email'),
        'width'     => '150',
        'index'     => 'email'
    ));

    $groups = Mage::getResourceModel('customer/group_collection')
        ->addFieldToFilter('customer_group_id', array('gt'=> 0))
        ->load()
        ->toOptionHash();

    $this->addColumn('group', array(
        'header'    =>  Mage::helper('customer')->__('Group'),
        'width'     =>  '100',
        'index'     =>  'group_id',
        'type'      =>  'options',
        'options'   =>  $groups,
    ));

    $this->addColumn('Telephone', array(
        'header'    => Mage::helper('customer')->__('Telephone'),
        'width'     => '100',
        'index'     => 'billing_telephone'
    ));

    $this->addColumn('billing_postcode', array(
        'header'    => Mage::helper('customer')->__('ZIP'),
        'width'     => '90',
        'index'     => 'billing_postcode',
    ));

    $this->addColumn('billing_country_id', array(
        'header'    => Mage::helper('customer')->__('Country'),
        'width'     => '100',
        'type'      => 'country',
        'index'     => 'billing_country_id',
    ));

    $this->addColumn('billing_region', array(
        'header'    => Mage::helper('customer')->__('State/Province'),
        'width'     => '100',
        'index'     => 'billing_region',
    ));

    $this->addColumn('customer_since', array(
        'header'    => Mage::helper('customer')->__('Customer Since'),
        'type'      => 'datetime',
        'align'     => 'center',
        'index'     => 'created_at',
        'gmtoffset' => true
    ));

    if (!Mage::app()->isSingleStoreMode()) {
        $this->addColumn('website_id', array(
            'header'    => Mage::helper('customer')->__('Website'),
            'align'     => 'center',
            'width'     => '80px',
            'type'      => 'options',
            'options'   => Mage::getSingleton('adminhtml/system_store')->getWebsiteOptionHash(true),
            'index'     => 'website_id',
        ));
    }

    $this->addColumn('action',
        array(
            'header'    =>  Mage::helper('customer')->__('Action'),
            'width'     => '100',
            'type'      => 'action',
            'getter'    => 'getId',
            'actions'   => array(
                array(
                    'caption'   => Mage::helper('customer')->__('Edit'),
                    'url'       => array('base'=> '*/*/edit'),
                    'field'     => 'id'
                )
            ),
            'filter'    => false,
            'sortable'  => false,
            'index'     => 'stores',
            'is_system' => true,
    ));

    $this->addExportType('*/*/exportCsv', Mage::helper('customer')->__('CSV'));
    $this->addExportType('*/*/exportXml', Mage::helper('customer')->__('Excel XML'));
    return parent::_prepareColumns();
}

protected function _prepareMassaction()
{
    $this->setMassactionIdField('entity_id');
    $this->getMassactionBlock()->setFormFieldName('customer');

    $this->getMassactionBlock()->addItem('delete', array(
         'label'    => Mage::helper('customer')->__('Delete'),
         'url'      => $this->getUrl('*/*/massDelete'),
         'confirm'  => Mage::helper('customer')->__('Are you sure?')
    ));

    $this->getMassactionBlock()->addItem('newsletter_subscribe', array(
         'label'    => Mage::helper('customer')->__('Subscribe to Newsletter'),
         'url'      => $this->getUrl('*/*/massSubscribe')
    ));

    $this->getMassactionBlock()->addItem('newsletter_unsubscribe', array(
         'label'    => Mage::helper('customer')->__('Unsubscribe from Newsletter'),
         'url'      => $this->getUrl('*/*/massUnsubscribe')
    ));

    $groups = $this->helper('customer')->getGroups()->toOptionArray();

    array_unshift($groups, array('label'=> '', 'value'=> ''));
    $this->getMassactionBlock()->addItem('assign_group', array(
         'label'        => Mage::helper('customer')->__('Assign a Customer Group'),
         'url'          => $this->getUrl('*/*/massAssignGroup'),
         'additional'   => array(
            'visibility'    => array(
                 'name'     => 'group',
                 'type'     => 'select',
                 'class'    => 'required-entry',
                 'label'    => Mage::helper('customer')->__('Group'),
                 'values'   => $groups
             )
        )
    ));

    return $this;
}

public function getGridUrl()
{
    return $this->getUrl('*/*/grid', array('_current'=> true));
}

public function getRowUrl($row)
{
    return $this->getUrl('*/*/edit', array('id'=>$row->getId()));
}

}

Its wrong way i know, but have to get just first name and don't have other way. But now this make my sort on columns from grid stop work. I need sort working. If i remove this 3 lines from my _prepareCollection(), it's work fine but i need it too and with it don't work my sort. I'm crazy with this someone can help???

Ty in advise.

You're best bet would be to load the firstname attribute on the collection and add that as an independent column. Adding column to customer grid in Magento - Data not populating will give you a good start. You should just need to add in ->addAttributeToSelect('firstname') and then add the column in the prepareColumns function...

$this->addColumn('firstname', array(
    'header'    => Mage::helper('customer')->__('First Name'),
    'index'     => 'firstname'
));