Search API Referencing entity relations
Implement hook_entity_property_info_alter()

function MY_MODULE_entity_property_info_alter(&amp;$info) {
    $info['node']['properties']['article_number'] = array(
        'type' =&gt; 'text',
        'label' =&gt; t('Article number'),
        'getter callback' =&gt; 'MY_MODULE_get_article_number',
    );
}

function MY_MODULE_get_article_number($node) {
    if ($node-&gt;type == 'product_page') {
        // Return the article number from the referenced product node
    }
}


Go to admin/config/search/search_api/index/{index_name}/fields and select the property. Now you can use this property like any other field.

In your case you need to configure this field in you view. I assume you added Search: Fulltext search to your filter criteria. Click on this filter and add the property to the Searched fields settings.

I hope this works for you.