module_definition.php

<?php

public function subscribeEvents() {
	Backend::$events->addEvent('shop:onExtendProductModel', $this, 'extend_product_model');
	Backend::$events->addEvent('shop:onExtendProductForm', $this, 'extend_product_form');

	Backend::$events->addEvent('shop:onOverrideProductCsvExportColumns', $this, 'set_product_csv_export_columns');
	Backend::$events->addEvent('shop:onOverrideProductCsvImportData', $this, 'set_product_csv_import_data');
}

public function extend_product_model($product) {
	$product->add_relation('belongs_to', 'artist', array(
		'class_name'  => 'StaveArtists_Artist',
		'foreign_key' => 'x_artist'
	));
	$product->define_relation_column('artist', 'artist', 'Artist', db_varchar, "concat(@last_name, ', ', @first_name)");
}

public function extend_product_form($product) {
	$product->add_form_field('artist', 'left')->tab('Puzzles')->emptyOption('<no artist selected>');
}

public function set_product_csv_export_columns($row_data) {
	$result = array('artist'=>null);

	if (!isset($row_data['x_artist']))
		return $result;

	if (!strlen($row_data['x_artist']))
		return $result;

	$result['artist'] = $row_data['x_artist'];

	return $result;
}

public function set_product_csv_import_data($row_data) {
	$result = array(
		'warnings' => array(),
		'data'     => array('x_artist' => null)
	);

	if (!isset($row_data['artist']))
		return $result;

	if (!strlen($row_data['artist']))
		return $result;

	$artist = StaveArtists_Artist::create()->find($row_data['artist']);

	if (!$artist) {
		$result['warnings'][] = sprintf('Artist "%s" is not found.', $row_data['artist']);
		return $result;
	}

	$result['data']['x_artist'] = $row_data['artist'];

	return $result;
}

?>

relation_table.sql

CREATE TABLE staveartists_artists (
  id int(11) NOT NULL auto_increment,
  created_at datetime default NULL,
  updated_at datetime default NULL,
  created_user_id int(11) default NULL,
  updated_user_id int(11) default NULL,
  last_name varchar(50) NOT NULL default '',
  first_name varchar(50) NOT NULL default '',
  featured tinyint(1) NOT NULL default 0,
  PRIMARY KEY  (id)
);

ALTER TABLE shop_products ADD column x_artist int;