Ajax Callbacks in Drupal Forms API

ajax_callback_drupal.php

<?php
function partners_field_attach_form($entity_type, $entity, &$form, &$form_state, $langcode) {
  if ($entity_type != 'field_collection_item' || $entity->field_name != 'field_partner_products') {
    return;
  }

  $form['#after_build'][] = 'partners_products_after_build';

  $id = drupal_html_id('partners_field');
  $form['#prefix'] = '<div id="'. $id .'">';
  $form['#suffix'] = '</div>';

  $form['field_partner_product'][LANGUAGE_NONE]['#ajax'] = array(
    'callback' => 'partners_field_ajax_callback',
    'wrapper' => $id,
  );
}

function partners_products_after_build($form, &$form_state) {
  if (empty($form['field_partner_product'][LANGUAGE_NONE]['#value'])) {
    $form['field_partner_specialisaties']['#access'] = FALSE;
    $form['field_partner_certificaties']['#access'] = FALSE;
  } else {
    if (is_array($form['field_partner_product'][LANGUAGE_NONE]['#value'])) {
      $value = !empty($form['field_partner_product'][LANGUAGE_NONE]['#value'][0]) ? $form['field_partner_product'][LANGUAGE_NONE]['#value'][0] : FALSE;
    } else {
      $value = $form['field_partner_product'][LANGUAGE_NONE]['#value'];
    }
    if (!empty($value)) {
      $tids = partners_get_tids_for_pid($value);
      $tids = $tids ? $tids : array();

      foreach (array('field_partner_specialisaties','field_partner_certificaties') as $spec_field) {
        if ($tids) {
          foreach (array_keys($form[$spec_field][LANGUAGE_NONE]['#options']) as $tid) {      
            if (!in_array($tid, $tids)) {
              // unset($form[$spec_field][LANGUAGE_NONE]['#options'][$tid]);
              if (!empty($form[$spec_field][LANGUAGE_NONE][$tid])) {
                $form[$spec_field][LANGUAGE_NONE][$tid]['#access'] = FALSE;
              }
            }
          }
        } else {
          $form[$spec_field][LANGUAGE_NONE]['#access'] = FALSE;
        }
      }
    }
  }


  return $form;
}

function partners_field_ajax_callback($form, $form_state) {  
  $delta = $form_state['triggering_element']['#parents'][2];

  hide($form['field_partner_products'][LANGUAGE_NONE][$delta]['_weight']);

  return $form['field_partner_products'][LANGUAGE_NONE][$delta];
}

function partners_get_tids_for_pid($product_id) {
  $partner_specialisaties = taxonomy_vocabulary_machine_name_load('partner_specialisaties');
  $partner_certificaties = taxonomy_vocabulary_machine_name_load('partner_certificaties');
  if (empty($product_id)) {
    return;
  }

  // Query all taxonomy terms with selected product id and Put access true on all of these
  $query = new EntityFieldQuery();
  $query->entityCondition('entity_type', 'taxonomy_term')
    ->propertyCondition('vid', array($partner_specialisaties->vid, $partner_certificaties->vid), 'IN' )
    ->fieldCondition('field_partner_product', 'value', $product_id, '=')
    ->range(0,10);
  $result = $query->execute();

  if (empty($result)) {
    return;
  }

  $tids = array();
  foreach ($result['taxonomy_term'] as $tid => $term) {
    $tids[] = $tid;
  }

  return $tids;
}