<?php echo $field->input; ?>

XxxxHelperForm::getItems()

prepareModel()

       _user_usergroup_map_id_.group_id
       _usergroup_id_.title

		$this->hasOne('user_usergroup_map_id', // name
			'.user_usergroup_map', // foreignModelClass
			'user_id', // localKey
			'user_id' // foreignKey
		);

		$this->hasOne('usergroup_id', // name
			'.usergroups', // foreignModelClass
			'user_usergroup_map_id.group_id', // localKey
			'id' // foreignKey
		);

	$this->orm->select(array(
		'user_id',
		'user_id.name',
		'user_usergroup_map_id.group_id',
		'usergroup_id.title'
	));

	<field name="users"
			alias="id"
			label="TEST_FIELD_NAME"
			filter="array"
			type="relation"
			multiple="true"
			labelKey="_user_id_name"/>

prepareModel(&$model)

	$model_created_by = CkJModel::getInstance('ThirdUsers', 'TestsModel');
	$model_created_by->addGroupOrder("a.username");
	$lists['fk']['created_by'] = $model_created_by->getItems();