Custom field type deletes data from table when I modify the page

Question

I'm using Drupal 7 and I have a problem with custom field type generated by field type generator.

I attached this field to a content type, and set his value to unlimited. When I create content type and add values it works fine. So, when I modify the page, values are deleted from database and I must add again. Here attach the functions generated by field type generator.

/**
 * @file
 * mediatrack module.
 * Media Track field.
 */

/**
 * Implements hook_field_info().
 */
function mediatrack_field_info() {
  return array(
    'mediatrack' => array(
      'label' => t('Mediatrack'),
      'description' => t('The media track'),
      'default_widget' => 'mediatrack_widget_default',
      'default_formatter' => 'mediatrack_formatter_default',
    ),
  );
}

/**
 * Implements hook_field_widget_info().
 */
function mediatrack_field_widget_info() {
  return array(
    'mediatrack_widget_default' => array(
      'label' => t('Default widget'),
      'field types' => array('mediatrack'),
      'behaviors' => array(
        'multiple values' => FIELD_BEHAVIOR_DEFAULT,
        'default value' => FIELD_BEHAVIOR_DEFAULT,
      ),
    ),
  );
}

/**
 * Implements hook_field_widget_form().
 */
function mediatrack_field_widget_form(&$form, &$form_state, $field, $instance, $langcode, $items, $delta, $element) {
  switch ($instance['widget']['type']) {
    case 'mediatrack_widget_default':
      $element['mediatrack']['trackid'] = array(
        '#type' => 'textfield',
        '#title' => t('Trackid'),
        '#default_value' => strval(next_trackid($field)), 
      );
      $element['mediatrack']['tracktype'] = array(
        '#type' => 'select',
        '#options' => array("---","audio","video"),
        '#title' => t('Tipo traccia'),
      );
      $element['mediatrack']['tracktitle'] = array(
        '#type' => 'textfield',
        '#title' => t('titolo'),
        '#maxlength' => 255,
        '#default_value' => isset($items[$delta]['tracktitle']) ? $items[$delta]['tracktitle'] : NULL,
      );
      $element['mediatrack']['trackdesc'] = array(
        '#type' => 'text_format',
        '#title' => t('Descrizione'),
        '#format' => isset($items[$delta]['trackdesc_format']) ? $items[$delta]['trackdesc_format'] : NULL,
        '#default_value' => isset($items[$delta]['trackdesc']) ? $items[$delta]['trackdesc'] : '',
      );
      $element['mediatrack']['tracksource'] = array(
        '#type' => 'textfield',
        '#title' => t('file'),
        '#maxlength' => 255,
        '#default_value' => isset($items[$delta]['tracksource']) ? $items[$delta]['tracksource'] : NULL,
      );
    // Collapse existing entries, for some UX.
    if (($delta < sizeof($items))) {
      $fieldset_title = t("{$instance['label']} #{$delta}");
    }
    else {
      $fieldset_title = t("New entry for {$instance['label']}");
    }
    $element += array('#type' => 'fieldset');
    $element['#title'] = t('@fieldset_title', array('@fieldset_title' => $fieldset_title));
    $element += array(
      '#collapsible' => TRUE,
      '#collapsed' => ($delta < sizeof($items)),
    );
    break;
  }
  return $element;
}


function next_trackid($field) {
  $fieldname = array_keys($field['storage']['details']['sql']['FIELD_LOAD_CURRENT'])[0];
  $name = $field['field_name']; 
  $id = db_query("select max(" . $name . "_trackid) from {" . $fieldname . "};")->fetchfield();
  if ($id == NULL) {
    return 1;
  }
  else {
    return $id+1;
  }
}

/**
 * Implements hook_field_validate().
 */
function mediatrack_field_validate($entity_type, $entity, $field, $instance, $langcode, $items, &$errors) {
  if ($field['type'] == 'mediatrack') {
    foreach ($items as $delta => $item) {
      // If field is not considered empty, validate its data.
      if (!mediatrack_field_is_empty($item, $field)) {
      }
    }
  }
}

/**
 * Implements hook_field_presave().
 */
function mediatrack_field_presave($entity_type, $entity, $field, $instance, $langcode, &$items) {
  // Structure the field data as expected by the database. Only act when the
  // elements come nested as 'publisher_reference'. Otherwise, no change needed.
  if (!empty($items) && $field['type'] == 'mediatrack' && array_key_exists('mediatrack', $items[0])) {
    foreach ($items as $delta => $item) {
      $items[$delta]['trackid'] = $item['mediatrack']['trackid'];
      $items[$delta]['tracktype'] = $item['mediatrack']['tracktype'];
      $items[$delta]['tracktitle'] = $item['mediatrack']['tracktitle'];
      $items[$delta]['trackdesc'] = $item['mediatrack']['trackdesc']['value'];
      $items[$delta]['trackdesc_format'] = $item['mediatrack']['trackdesc']['format'];
      $items[$delta]['tracksource'] = $item['mediatrack']['tracksource'];
    }
  }
}

/**
 * Implements hook_field_formatter_info().
 */
function mediatrack_field_formatter_info() {
  return array(
    'mediatrack_formatter_default' => array(
      'label' => t('Default formatter'),
      'field types' => array('mediatrack'),
    ),
  );
}

This is a mediatrack field. Trackid is set to trackid +1 by default; I have a textfield for test only. When I modify, I can see table with previously added field collapsed, but, when I save, I don't see these fields, and the table is empty, if I don't add anything.

Do I need to set something?

Answer 1

I solved setting 'not null' to FALSE in my field_schema function, to trackid. Now it works!