Sonata AdminBundle: How to create a custom field

I wanted to create a custom field that is not in the database for the listFields so I searched the web how do it. Here is how I have done it:

class Entity 
{
    ...
    //custom getter
    public function getFullName()
    {
        return $this->getFirstName() . ' ' . $this->getLastName();
    }
}

In your EntityAdmin:

 protected function configureListFields(ListMapper $listMapper)
    {
        $listMapper
           ->add('fullName', 'doctrine_orm_string')
            ->add('_action', 'actions', array(
                'actions' => array(
                    'view' => array(),
                    'edit' => array(),
                    'delete' => array(),
                )
            ))
        ;
    }