<?php
   class Course extends DataObject {
      static $db = array(
         "Name" => "Varchar",
         "Number" => "Int",
         "Description" => "Text",
         "Active" => "Boolean"
      );
      
      static $has_one = array(
         "Category" => "CourseCategory"
      );
      
      public function getCMSFields_forPopup()
      {
         $categories = DataObject::get("CourseCategory");
         $dropdown = new DropdownField('Category');
         
         if ($categories) {
            $map = $categories->toDropDownMap('ID', 'Name');   
            $dropdown = new DropdownField('CategoryID', 'Category', $map);
         }
         
         return new FieldSet(
            $dropdown,
            new TextField('Name'),
            new TextField('Number'),
            new TextareaField('Description'),
            new CheckboxField('Active')
         );
      }
   }

<?php
class CoursesPage extends StandardPage {

   static $has_many = array(
      "Courses" => "Course"
   );

   public function getCMSFields() {
      $fields = parent::getCMSFields();
      
      $course_fields = new DataObjectManager(
         $this,
         "Courses",
         "Course",
         array("Category.Name" => "Category", "Number" => "Number","Name" => "Name", "Description" => "Description", "Active" => "Active"),
         "getCMSFields_forPopup"
      );
      
      $course_category_fields = new DataObjectManager(
         $this,
         "Categories",
         "CourseCategory",
         array("Name" => "Name"),
         "getCMSFields_forPopup"
      );

      $fields->addFieldToTab("Root.Content.Courses", $course_fields);
      $fields->addFieldToTab("Root.Content.Categories", $course_category_fields);
      
      return $fields;
   }

}

class CoursesPage_Controller extends StandardPage_Controller {

   public function init() {
      parent::init();
      Requirements::themedCSS('courses');
   }
   
}
?>

Error at sapphire/core/model/MySQLDatabase.php line 525: Couldn't run query:
SELECT "Course"."ClassName", "Course"."Created", "Course"."LastEdited", "Course"."Name", "Course"."Number", "Course"."Description", "Course"."Active", "Course"."CategoryID", "Course"."ID", CASE WHEN "Course"."ClassName" IS NOT NULL THEN "Course"."ClassName" ELSE 'Course' END AS "RecordClassName"
FROM "Course"
WHERE ("ParentID" = '11')

Unknown column 'ParentID' in 'where clause' ({SITE_URL}/courses)