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Introduction

In some situations, it makes sense to index a numeric representation of a string attribute, for example for sorting by clothing size. Size is usually a string (S, M, L, etc), but sorting alphabetically will yield strange results (S, L, M). It makes sense to replace these values with a numeric attribute to sort. It is possible to add a new attribute containing this value by implementing a custom observer. To make changes to the attributes being indexed, observe the algolia_after_create_product_object event with a custom observer.
This tutorial assumes knowledge of writing a custom observer. More info can be found here.

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Create observer

Create the Observer/TransformAttribute.php file, and add a new Observer class in it.
php
namespace Algolia\CustomAlgolia\Observer;

use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;

class TransformAttribute implements ObserverInterface
{
  public function execute(Observer $observer)
  {
    $record = $observer->getData('custom_data');

    // $sizes = ['S', 'M', 'L'];
    $sizes = $record['sizes'];

    // S => 1, M => 2, L => 3
    $replacementNumbers = [1, 2, 3];

    $record['numeric_sizes'] = array_replace($sizes, $replacementNumbers);
  }
}
The code in the execute block can be modified as needed. In this example, the clothing sizes are transformed into numeric values.

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Register the observer

To register your observer, add the following snippet to the etc/events.xml file:
xml
<event name="algolia_after_create_product_object">
    <observer name="customalgolia_transform_attribute" instance="Algolia\CustomAlgolia\Observer\TransformAttribute" />
</event>

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Reindex data

With the custom observer in place, the catalog needs to be reindexed in order to for the changes to take effect. Read more about (re)indexing here.