Adding new field to donations export based on custom campaign field

function.php

<?php
/**
 * In this example, we add a custom field to campaigns, which is
 * automatically included in the Donations export for donations
 * to those campaigns.
 */
add_action(
	'init',
	function() {
		/**
		 * Step 1: Register the campaign field.
		 *
		 * This will add a new field when you are editing a
		 * campaign allowing you to define the Fund Code for
		 * that campaign.
		 */
		$campaign_field = new Charitable_Campaign_Field(
			'Fund_Code',
			array(
				'label'         => 'Fund Code',
				'data_type'     => 'meta',
				'admin_form'    => array(
					'type'     => 'text',
					'required' => false,
				),
				'show_in_export' => true,
				'value_callback' => false,
			)
		);

		charitable()->campaign_fields()->register_field( $campaign_field );

		/**
		 * Step 2: Register the donation field.
		 *
		 * This will add it to the donations export. For each
		 * donation, it will look up the "Fund Code" for the campaign
		 * that received the donation, and add that in its own column.
		 */
		$donation_field = new Charitable_Donation_Field(
			'Fund_Code',
			array(
				'label'          => 'Fund Code',
				'data_type'      => 'core',
				'donation_form'  => false,
				'admin_form'     => false,
				'show_in_export' => true,
				'value_callback' => function( Charitable_Donation $donation ) {
					$campaign_id = current( $donation->get_campaign_donations() )->campaign_id;
					return charitable_get_campaign( $campaign_id )->get( 'Fund_Code' );
				},
			)
		);

		charitable()->donation_fields()->register_field( $donation_field );
	}
);