set_custom_fields_value() inside of /models/posts.php for ACF

gistfile1.php

<?php
function set_custom_fields_value() {

    global $json_api;

    if ($json_api->include_value('custom_fields') && $json_api->query->custom_fields) {

        // Query string params for this query var
        $params = trim($json_api->query->custom_fields);

        // Get all custom fields if true|all|* is passed
        if ($params === "*" || $params === "true" || $params === "all") {

            $wp_custom_fields = get_post_custom($this->id);
            $this->custom_fields = new stdClass();

            // Loop through our custom fields and place on property
            foreach($wp_custom_fields as $key => $val) {
                if (get_field($key)) {
                    $this->custom_fields->$key = get_field($key);
                } else if ($val) {
                    // Some fields are stored as serialized arrays.
                    // This method does not support multidimensionals... 
                    // but didn't see anything wrong with this approach
                    $current_custom_field = @unserialize($wp_custom_fields[$key][0]);

                    if (is_array($current_custom_field)) {

                        // Loop through the unserialized array
                        foreach($current_custom_field as $sub_key => $sub_val) {

                            // Lets append these for correct JSON output
                            $this->custom_fields->$key->$sub_key = $sub_val;
                        }

                    } else {

                        // Break this value of this custom field out of its array
                        // and place it on the stack like usual
                        $this->custom_fields->$key = $wp_custom_fields[$key][0];

                    }
                }
            }
        } else {

            // Well this is the old way but with the unserialized array fix
            $params = explode(',', $params);
            $wp_custom_fields = get_post_custom($this->id);
            $this->custom_fields = new stdClass();

            foreach ($params as $key) {

                if (isset($wp_custom_fields[$key]) && $wp_custom_fields[$key][0] ) {
                    $current_custom_field = @unserialize($wp_custom_fields[$key][0]);

                    if (is_array($current_custom_field)) {
                        foreach($current_custom_field as $sub_key => $sub_val) {
                            $this->custom_fields->$key->$sub_key = $sub_val;
                        }

                    } else {
                        $this->custom_fields->$key = $wp_custom_fields[$key][0];

                    }

                }
            }
        }

    } else {
        unset($this->custom_fields);

    }
}