public function add_attendee_data( $order_id, $contact_id ) {

        $order = wc_get_order( $order_id );

        foreach ( $this->get_attendees_from_order( $order ) as $attendee ) {

            $settings = get_post_meta( $attendee['WooCommerceEventsProductID'], 'wpf-settings-woo', true );

            if ( empty( $settings ) || ! isset( $settings['add_attendees'] ) || $settings['add_attendees'] != true ) {
                continue;
            }

            // This was already sent in the main order data so it doesn't need to be sent again

            if ( $attendee['WooCommerceEventsAttendeeEmail'] == $order->get_billing_email() ) {
                continue;
            }

            if ( empty( $attendee['WooCommerceEventsAttendeeEmail'] ) ) {
                wpf_log( 'notice', 0, 'Unable to sync attendee data, no email address provided. To sync attendees you must enable <strong>Capture attendee full name and email address?</strong> when editing the FooEvent product.' );
                continue;
            }

            $update_data = array(
                'first_name'           => $attendee['WooCommerceEventsAttendeeName'],
                'attendee_first_name'  => $attendee['WooCommerceEventsAttendeeName'],
                'last_name'            => $attendee['WooCommerceEventsAttendeeLastName'],
                'attendee_last_name'   => $attendee['WooCommerceEventsAttendeeLastName'],
                'user_email'           => $attendee['WooCommerceEventsAttendeeEmail'],
                'attendee_email'       => $attendee['WooCommerceEventsAttendeeEmail'],
                'billing_phone'        => $attendee['WooCommerceEventsAttendeeTelephone'],
                'phone_number'         => $attendee['WooCommerceEventsAttendeeTelephone'],
                'attendee_phone'       => $attendee['WooCommerceEventsAttendeeTelephone'],
                'billing_company'      => $attendee['WooCommerceEventsAttendeeCompany'],
                'company'              => $attendee['WooCommerceEventsAttendeeCompany'],
                'attendee_company'     => $attendee['WooCommerceEventsAttendeeCompany'],
                'attendee_designation' => $attendee['WooCommerceEventsAttendeeDesignation'],
            );

            // Merge event and venue fields
            $product_id = $attendee['WooCommerceEventsProductID'];

            $hour    = get_post_meta( $product_id, 'WooCommerceEventsHour', true );
            $minutes = get_post_meta( $product_id, 'WooCommerceEventsMinutes', true );
            $period  = get_post_meta( $product_id, 'WooCommerceEventsPeriod', true );

            $event_fields = array(
                'event_name'       => get_the_title( $product_id ),
                'event_start_date' => get_post_meta( $product_id, 'WooCommerceEventsDate', true ),
                'event_start_time' => $hour . ':' . $minutes . ' ' . $period,
                'event_venue_name' => get_post_meta( $product_id, 'WooCommerceEventsLocation', true ),
                'zoom_meeting_id'  => get_post_meta( $product_id, 'WooCommerceEventsZoomWebinar', true ),
                'zoom_join_url'    => get_post_meta( $product_id, 'wp_fusion_zoom_join_url', true ),
            );

            // Zoom.

            if ( ! empty( $event_fields['zoom_meeting_id'] ) && empty( $event_fields['zoom_join_url'] ) && wpf_is_field_active( 'zoom_join_url' ) && class_exists( 'FooEvents_Zoom_API_Helper' ) ) {

                // The Zoom integration currently doesn't cache the meeting URL in the database so we'll fetch it one time here.

                $helper = new FooEvents_Zoom_API_Helper();
                $result = $helper->do_fooevents_fetch_zoom_meeting( $event_fields['zoom_meeting_id'] );

                if ( ! empty( $result['status'] ) && 'success' === $result['status'] ) {
                    $event_fields['zoom_join_url'] = $result['data']['join_url'];
                    update_post_meta( $product_id, 'wp_fusion_zoom_join_url', $event_fields['zoom_join_url'] );
                }
            }

            // Bookings extension.

            if ( ! empty( $attendee['WooCommerceEventsBookingOptions'] ) ) {

                $slot = $attendee['WooCommerceEventsBookingOptions']['slot'];
                $date = $attendee['WooCommerceEventsBookingOptions']['date'];

                $booking_options = get_post_meta( $product_id, 'fooevents_bookings_options_serialized', true );
                $booking_options = json_decode( $booking_options, true );

                if ( ! empty( $booking_options ) && isset( $booking_options[ $slot ] ) ) {

                    $time = trim( $booking_options[ $slot ]['formatted_time'], '()' );
                    $date = $booking_options[ $slot ]['add_date'][ $date ]['date'];

                    $event_fields['booking_date'] = $date . ' ' . $time;
                    $event_fields['booking_time'] = $time;

                }
            }

            $update_data = array_merge( $update_data, $event_fields );

            // Merge custom fields
            if ( ! empty( $attendee['WooCommerceEventsCustomAttendeeFields'] ) ) {

                // New v5.5+ method

                $update_data = array_merge( $update_data, $attendee['WooCommerceEventsCustomAttendeeFields'] );

                // Old method:

                foreach ( $attendee['WooCommerceEventsCustomAttendeeFields'] as $key => $value ) {

                    $key = strtolower( str_replace( 'fooevents_custom_', '', $key ) );

                    $update_data[ $key ] = $value;

                }
            }

            $update_data = apply_filters( 'wpf_woocommerce_attendee_data', $update_data, $attendee, $order_id );

            $contact_id = wp_fusion()->crm->get_contact_id( $update_data['user_email'] );

            wpf_log( 'info', 0, 'FooEvents adding new event attendee for order <a href="' . admin_url( 'post.php?post=' . $order_id . '&action=edit' ) . '" target="_blank">#' . $order_id . '</a>:', array( 'meta_array' => $update_data ) );

            if ( empty( $contact_id ) ) {

                $contact_id = wp_fusion()->crm->add_contact( $update_data );

            } else {

                wp_fusion()->crm->update_contact( $contact_id, $update_data );

            }

            $apply_tags = array();

            // Product settings

            if ( ! empty( $settings['apply_tags_event_attendees'] ) ) {
                $apply_tags = array_merge( $apply_tags, $settings['apply_tags_event_attendees'] );
            }

            // Variation settings

            if ( ! empty( $attendee['WooCommerceEventsVariationID'] ) ) {

                $settings = get_post_meta( $attendee['WooCommerceEventsVariationID'], 'wpf-settings-woo', true );

                if ( ! empty( $settings ) && ! empty( $settings['apply_tags_event_attendees_variation'] ) && ! empty( $settings['apply_tags_event_attendees_variation'][ $attendee['WooCommerceEventsVariationID'] ] ) ) {
                    $apply_tags = array_merge( $apply_tags, $settings['apply_tags_event_attendees_variation'][ $attendee['WooCommerceEventsVariationID'] ] );
                }
            }

            if ( ! empty( $apply_tags ) ) {

                wpf_log( 'info', 0, 'Applying tags to FooEvents attendee for contact ID ' . $contact_id . ': ', array( 'tag_array' => $apply_tags ) );

                wp_fusion()->crm->apply_tags( $apply_tags, $contact_id );

            }
        }

    }