Drupal: Add custom field to Search API Solr index

custom_index.php

<?php
/**
 * Implements hook_entity_property_info_alter().
 */
function example_search_api_property_entity_property_info_alter(&$info) {
  $info['node']['properties']['created_week_day'] = array(
    'type' => 'text',
    'label' => t('Week day of node creation'),
    'sanitized' => TRUE,
    'getter callback' => 'example_search_api_property_weekday_getter_callback',
  );
  $info['node']['properties']['test_multiple_field'] = array(
    'type' => 'list<text>',
    'label' => t('Test multiple text'),
    'sanitized' => TRUE,
    'getter callback' => 'example_search_api_property_random_text_getter_callback',
  );
}

/**
 * Getter callback for created_week_day property.
 */
function example_search_api_property_weekday_getter_callback($item) {
  return format_date($item->created, 'custom', 'D');
}
 
/**
 * Getter callback for multiple field.
 */
function example_search_api_property_random_text_getter_callback($item) {
  $strings = array('one', 'two', 'three', 'four', 'five');
  $number = rand(1, 5);
 
  $values = array();
  while ($number > 0) {
    $values[] = $strings[rand(0, 4)];
    $number--;
  }
 
  return $values;
}