Adding fields to the WordPress REST API

The WordPress REST API is an incredibly powerful tool; it allows us to use the CMS in exciting ways. In particular it allows us to use reactive frameworks for our front-end development. For example, we could use WordPress as a back-end (headless) and a reactive framework like Vue.js or React as our front-end. We could also create an app which uses data stored in WordPress on our mobile devices or even access data using calls in our custom plugins.

Extending the WordPress API

Out of the box the API gives us access to several useful fields (https://[yourdomain.com]/wp-json/wp/v2/posts). I expect that you’ll want to extend these, though. For example, we don’t have access to the post author name or feature image url by default. Let’s have a look at extending the the API to include these.

Register a rest field

To register new rest fields we use the register_rest_field() function. This field takes 3 arguments.

The first argument can be a string or an array. It allows us to define the content types we would like to register the field to. In this example we will registering from a ‘post’.

The second argument gives the field a name. This should just be a string.

The third argument is an array of arguments that we can use to populate the field we are registering. In this example we are going to use a ‘callback’ function to pass specific values to our new field.

register_rest_field(array('post'), 'gb_author_name', array('get_callback' => 'gb_get_author_name'));

register_rest_field(array('post'), 'gb_feature_image', array('get_callback' => 'gb_get_featured_image'));

Passing the author name

We are going to use a ‘callback’ function to pass the author name into our new rest field. We start by declaring our function.

function gb_get_author_name($object, $field_name, $request) {

}

Then we want to check when we are dealing with the author object and return the display name using get_the_author_meta().

function gb_get_author_name($object, $field_name, $request) {
    if ($object['author']) {
        return get_the_author_meta('display_name', $object['author']);
    } else {
        return;
    }
}

Passing the featured image url and alt text

Next we will look at how to use a ‘callback’ function to pass an array consisting of the featured image url and alt text.

Again we start by declaring our ‘callback’ function.

function gb_get_featured_image($object, $field_name, $request) {

}

This time we want to check whether the object is the featured media. We can now grab the image url using wp_get_attachment_image_src() and the alt text with get_post_meta() and return an array of the results.

function gb_get_featured_image($object, $field_name, $request) {
    if ($object['featured_media']) {
        // Get the image url and the post meta from the featured media ID
        $image = wp_get_attachment_image_src($object['featured_media'], 'full');
        $alt = get_post_meta($object['featured_media'], '_wp_attachment_image_alt', true);

        return array('url' => $image[0], 'alt' => $alt);
    } else {
        return;
    }
}

Advanced Custom Fields

I frequently use the Advanced Custom Fields pro (ACF) plugin. I find this useful because it allows me to make flexible fields quickly. Luckily there is a handy plugin to expose any ACF fields to the rest API. You can download it here.