Drupal form attach one field

Drupal_form_attach_field.php

<?php
function wj_users_reg_interests_form($form = array(), &$state) {
  $account = user_load($GLOBALS['user']->uid);
  field_attach_form('user', $account, $form, $state, NULL, array('field_name' => 'field_interests'));

  // Add submit button.
  $form['actions'] = array(
    '#type' => 'actions',
    'submit' => array(
      '#type' => 'submit',
      '#value' => t('Next Step'),
      '#attributes' => array(
        'class' => array('btn btn-primary btn-lg pull-right'),
      ),
    ),
  );

  // Overwrite all other submit handlers, we'll run them manually in submit handler
  $form['#submit'] = array(
    'wj_users_reg_interests_form_submit'
  );
  
  return $form;
}

function wj_users_reg_interests_form_submit($form, &$form_state) {
  $account = $form['field_interests'][LANGUAGE_NONE]['#entity'];
  // Execute submit hook, removes empty fields.
  field_attach_submit('user', $account, $form, $form_state);
  // Populate $edit with the properties of $account, which have been edited on
  // this form by taking over all values, which appear in the form values too.
  $edit = array_intersect_key((array) $account, $form_state['values']);
  $account = user_save($account, $edit);
}