source: sipes/modules_contrib/nodereference_autocreate/nodereference_autocreate.module @ a8b1f3f

<?php

/**
 * @file
 * Defines a nodereference widget for auto-creating nodes. The widget
 * auto-creates a node and then updates its title on change.
 */

/**
 * Implementation of hook_widget_info().
 */
function nodereference_autocreate_widget_info() {
  return array(
    'nodereference_autocreate' => array(
      'label' => t('Auto-create node'),
      'field types' => array('nodereference'),
      'multiple values' => CONTENT_HANDLE_CORE,
      'callbacks' => array(
        'default value' => CONTENT_CALLBACK_DEFAULT,
      ),
    ),
  );
}

/**
 * Implementation of hook_widget().
 */
function nodereference_autocreate_widget(&$form, &$form_state, $field, $items, $delta = 0) {
  switch ($field['widget']['type']) {
    case 'nodereference_autocreate':
      $element = array(
        '#type' => 'text_textfield',
        '#default_value' => isset($items[$delta]) ? $items[$delta] : NULL,
        '#value_callback' => 'nodereference_autocreate_value',
        '#process' => array('nodereference_autocreate_process'),
      );
      break;
  }
  return $element;
}

/**
 * Value for a nodereference auto-create element.
 *
 * Substitute in the node title for the node ID.
 */
function nodereference_autocreate_value($element, $edit = FALSE) {
  $field_key = $element['#columns'][0];
  if ($nid = $element['#default_value'][$field_key]) {
    $value = db_result(db_query(db_rewrite_sql('SELECT n.title FROM {node} n WHERE n.nid = %d'), $nid));
    return array($field_key => $value);
  }
  return array($field_key => NULL);
}

/**
 * Process an individual element.
 *
 * @see nodereference_autocomplete_process()
 */
function nodereference_autocreate_process($element, $edit, $form_state, $form) {
  $field_key = $element['#columns'][0];

  $element[$field_key] = array(
    '#type' => 'text_textfield',
    '#default_value' => isset($element['#value']) ? $element['#value'] : '',
    // The following values were set by the content module and need to be
    // passed down to the nested element.
    '#title' => $element['#title'],
    '#required' => $element['#required'],
    '#description' => $element['#description'],
    '#field_name' => $element['#field_name'],
    '#type_name' => $element['#type_name'],
    '#delta' => $element['#delta'],
    '#columns' => $element['#columns'],
  );
  if (empty($element[$field_key]['#element_validate'])) {
    $element[$field_key]['#element_validate'] = array();
  }
  array_unshift($element[$field_key]['#element_validate'], 'nodereference_autocreate_validate');

  return $element;
}

/**
 * Validate an individual element.
 *
 * Create a node with the specified value on first save. Update title of
 * referenced node on each additional save.
 */
function nodereference_autocreate_validate($element, &$form_state) {
  $field_name = $element['#field_name'];
  $type_name = $element['#type_name'];
  $field = content_fields($field_name, $type_name);
  $field_key  = $element['#columns'][0];
  $delta = $element['#delta'];
  $value = $element['#value'][$field_key];
  $nid = NULL;

  if ($value) {
    // Load the current value stored in the database.
    $self = node_load($form_state['values']['nid']);
    $current_nid = $self->$field_name;
    $current_nid = $current_nid[$delta]['nid'];

    if ($current_nid) {
      // Referenced node exists, update its title if changed.
      $referenced_node = node_load($current_nid);
      if ($referenced_node->title != $value) {
        $referenced_node->title = $value;
        node_save($referenced_node);
      }

      $nid = $referenced_node->nid;
    }
    else {
      // Create a new node with the specified title.
      $node = new stdClass();
      $node->type = nodereference_autocreate_type_get($field['referenceable_types']);
      $node->title = $value;
      node_save($node);

      if (!$node->nid) {
        form_error($element[$field_key], t('%name: failed to create sub node.', array('%name' => t($field['widget']['label']))));
      }
      else {
        $nid = $node->nid;
      }
    }
  }
  form_set_value($element, $nid, $form_state);
}

/**
 * Implementation of hook_form_FORM_ID_alter(): content_field_edit_form.
 */
function nodereference_autocreate_form_content_field_edit_form_alter(&$form, $form_state) {
  if ($form['#field']['widget']['type'] == 'nodereference_autocreate') {
    $form['field']['referenceable_types']['#description'] .= t('Only one type can be referenced when using the ' .
      'auto-create node widget. The node type referenced will be used when creating a node.');

    $form['#validate'][] = 'nodereference_autocreate_content_field_edit_form_validate';
  }
}

/**
 * Validate nodereference setting form.
 *
 * If widget type is nodereference_autocreate then check for multiple selected
 * referenceable types.
 */
function nodereference_autocreate_content_field_edit_form_validate($form, &$form_state) {
  if ($form_state['values']['widget_type'] == 'nodereference_autocreate') {
    $count = 0;
    if ($form_state['values']['referenceable_types']) {
      foreach ($form_state['values']['referenceable_types'] as $type => $enabled) {
        if ($enabled) {
          $count++;
        }
      }

      if ($count > 1) {
        form_set_error('referenceable_types', t('Only one content type can be referenced for widgets of type <em>Auto-create node</em>.'));
      }
    }
  }
}

/**
 * Get the referenced type.
 *
 * @param $referenceable_types
 *   Referenceable types from field form.
 * @return
 *   Referenced type.
 */
function nodereference_autocreate_type_get(array $referenceable_types) {
  foreach ($referenceable_types as $type => $enabled) {
    if ($enabled) {
      return $type;
    }
  }
  return NULL;
}