moving tv's to create/edit document tag

<?php
/**
 * Register a form field to forms
 */

switch ($modx->event->name) {
    case 'OnDocFormPrerender':
        /* if you want to add custom scripts, css, etc, register them here */
        break;
    case 'OnDocFormRender':

        /* Get the TV */
        $tv = $modx->getObject('modTemplateVar',array('name'=>'mp3file'));

        $v = '';
        if (isset($scriptProperties['resource'])) {
            /* on the update screen, so set the value */
            /* get the raw content of the TV */
            $v = $tv->getValue($id);
        }
        /* now do the HTML */
        $fields = '
<div class="x-form-item x-tab-item">
    <label class="x-form-item-label">MP3 File</label>
    <div class="x-form-element" style="padding-left:205px;">
        <input type="text" name="mp3_file" value="'.$v.'" class="x-form-text x-form-field" />
    </div>
</div>
';
        $modx->event->output($fields);
break;
    case 'OnDocFormSave':
        /* do processing logic here. */
        $resource =& $scriptProperties['resource'];
        $resource->set('mp3file',$_POST['mp3_file']);
        $resource->save();
        break;
}
return;
?>

    case 'OnDocFormSave':
        /* do processing logic here. */
        $resource =& $scriptProperties['resource'];
        $resource->set('mp3file',$_POST['mp3_file']);
        $resource->save();
        break;
}
return;

$doc = $modx->getObject('modDocument',$resource->get('id'));

$tv = $modx->getObject('modTemplateVar', array('name' => 'mp3file'));
$tv->setValue($doc->get('id'), $_POST['mp3_file']);
$tv->save();