performance-art.fr/sites/all/modules/contrib/fields/content_taxonomy/content_taxonomy.module

<?php

/**
 * Implements hook_form_ID_alter().
 */
function content_taxonomy_form_field_ui_field_edit_form_alter(&$form, &$form_state, $form_id) {
  $field = $form['#field'];
  $instance = $form['#instance'];

  // Add parent selector to term reference fields,
  // except to the autocomplete widget, as it ignores the parent setting.
  if ($field['type'] == 'taxonomy_term_reference'
    && !($instance['widget']['type'] == 'taxonomy_autocomplete' || $instance['widget']['type'] == 'autocomplete_deluxe_taxonomy')) {

    // add parent form.
    foreach ($field['settings']['allowed_values'] as $delta => $tree) {
      $options[0] = '---';
      // todo this might break with huge vocs
      $voc = taxonomy_vocabulary_machine_name_load($tree['vocabulary']);
      foreach (taxonomy_get_tree($voc->vid) as $term) {
        $options[$term->tid] = str_repeat('- ', $term->depth) . $term->name;
      }

      $form['field']['settings']['allowed_values'][$delta]['parent']  = array(
        '#type' => 'select',
        '#title' => t('Parent'),
        '#options' => $options,
        '#default_value' => isset($tree['parent']) ? $tree['parent'] : 0,
      );

      $form['field']['settings']['allowed_values'][$delta]['depth']  = array(
        '#type' => 'textfield',
        '#title' => t('Tree depth'),
        '#default_value' => isset($tree['depth']) ? $tree['depth'] : '',
        '#description' => t('Set the depth of the tree. Leave empty to load all terms.'),
        '#element_validate' => array('_element_validate_integer_positive'),
      );
    }
  }

  // Add opt group setting.
  if ($field['type'] == 'taxonomy_term_reference' && $instance['widget']['type'] == 'options_select') {
    $form['instance']['widget']['settings']['content_taxonomy_opt_groups']  = array(
      '#type' => 'checkbox',
      '#title' => t('Render parent terms as opt-groups'),
      '#default_value' => isset($instance['widget']['settings']['content_taxonomy_opt_groups']) ? $instance['widget']['settings']['content_taxonomy_opt_groups'] : FALSE,
      '#description' => t('This option only works if you have a 2-level hierarchy in your vocabulary. Then the parents in the first level get opt-groups and the child terms will be selectable.'),
    );
  }
}

/**
 * Implements hook_field_info_alter().
 */
function content_taxonomy_field_info_alter(&$info) {
  // Use own options callback for handling additional configuration options.
  $info['taxonomy_term_reference']['settings']['options_list_callback'] = 'content_taxonomy_allowed_values';

  // Add depth option.
  foreach ($info['taxonomy_term_reference']['settings']['allowed_values'] as $key => $values) {
    $info['taxonomy_term_reference']['settings']['allowed_values'][$key]['depth'] = 0;
  }
}

/**
 * Returns the set of valid terms for a taxonomy field.
 * Extends taxonomy_allowed_values() with the tree depth option.
 *
 * @param $field
 *   The field definition.
 * @return
 *   The array of valid terms for this field, keyed by term id.
 */
function content_taxonomy_allowed_values($field) {
  $options = array();
  foreach ($field['settings']['allowed_values'] as $tree) {
    if ($vocabulary = taxonomy_vocabulary_machine_name_load($tree['vocabulary'])) {
      $max_depth = (isset($tree['depth']) && !empty($tree['depth'])) ? $tree['depth'] : NULL;
      if ($terms = taxonomy_get_tree($vocabulary->vid, $tree['parent'], $max_depth)) {
        foreach ($terms as $term) {
          $options[$term->tid] = str_repeat('- ', $term->depth) . $term->name;
        }
      }
    }
  }
  return $options;
}

/**
 * Implements hook_field_widget_info_alter().
 */
function content_taxonomy_field_widget_info_alter(&$info) {
  if (isset($info['options_select']['settings'])) {
    $info['options_select']['settings'] += array(
      'content_taxonomy_opt_groups' => FALSE,
    );
  }
}

/**
 * Implements hook_field_widget_form_alter().
 */
function content_taxonomy_field_widget_form_alter(&$element, &$form_state, $context) {
  $field = $context['field'];
  $instance = $context['instance'];

  if (!empty($instance['widget']['settings']['content_taxonomy_opt_groups'])) {
    $options = content_taxonomy_allowed_values_opt_groups($field);
    if (isset($element['#options']['_none'])) {
      $options = array('_none' => $element['#options']['_none']) + $options;
    }
    $element['#options'] = $options;
  }
}

/**
 * Helper function for generating opt groups.
 *
 * Similar to content_taxonomy_allowed_values(), but unfortunately we cannot
 * directly change content_taxonomy_allowed_values() as it only has the field
 * variable and opt groups are settings on the instance level. Still, this is
 * not a big performance issue, as taxonomy_get_tree statically caches some
 * data.
 */
function content_taxonomy_allowed_values_opt_groups($field) {
  $options = array();
  foreach ($field['settings']['allowed_values'] as $tree) {
    if ($vocabulary = taxonomy_vocabulary_machine_name_load($tree['vocabulary'])) {
      if ($terms = taxonomy_get_tree($vocabulary->vid, 0, 2)) {
        $current_group_term = NULL;
        foreach ($terms as $term) {
          if ($term->depth == 0) {
            $current_group_term = $term;
          }
          elseif ($term->depth == 1 && !is_null($current_group_term)) {
            $options[$current_group_term->name][$term->tid] = $term->name;
          }
        }
      }
    }
  }
  return $options;
}