Remove some meta fields from the ElasticPress/Elasticsearch index

elasticpress-remove-some-meta.php

<?php
add_filter(
	'ep_prepare_meta_data',
	function ( $prepared_meta ) {
		foreach ( $prepared_meta as $key => $meta_val ) {
			// Remove empty meta fields.
			if ( empty( $key ) || empty( $meta_val ) || empty( $meta_val[0] ) ) {
				unset( $prepared_meta[ $key ] );
				continue;
			}
			// Remove fields with key starting with "field_".
			if ( 0 === strpos( $meta_val[0], 'field_' ) ) {
				unset( $prepared_meta[ $key ] );
				continue;
			}
			// Remove fields with just letters, numbers, hyphens and underscores.
			if ( '_sku' !== $key && preg_match( '/^([a-zA-Z0-9\-\_]*)$/', $meta_val[0] ) ) {
				unset( $prepared_meta[ $key ] );
				continue;
			}
		}
		return $prepared_meta;
	}
);

This code is useful when you notice your content is not being fully indexed on ElasticPress/Elasticsearch. If you run
wp elasticpress index --setup --show-bulk-errors
and receive the following error
[illegal_argument_exception] Limit of total fields [5000] in index [<index_name>-post-1] has been exceeded
you need to reduce your meta fields number per post.

The code of this gist will remove all empty meta fields, fields with a value starting with "field_", and fields with a value containing only letters, numbers, hyphens, and underscores (probably just a code or hash).