class Customer extends DataObjectDecorator {
   
   static $summary_fields = array(
      'CompanyID' => 'Company'
   );
   
   function extraDBFields() {
      return array(
         'has_one' => array(
            'Company' => 'Company'
         )
      );
   }
   
   public function updateCMSFields(FieldSet &$fields) {
   
      $companies = DataObject::get("Company")->toDropdownMap('ID', 'CompanyName');
      $fields->push(new DropdownField("CompanyID", "Company", $companies));
}

public function updateSummaryFields(&$fields)
{
      $companies = DataObject::get("Company")->toDropdownMap('ID', 'CompanyName');
      $fields->push(new DropdownField("CompanyID", "Company", $companies));
}
}