Hi.
Someone probably solved this before, but I failed at searching the forum for this.
Let's have a look at this piece of code:

class MyObject extends DataObject {
  private static $has_many = array(
    'Objects' => 'CommonObject',
  );
}


class MyPage extends Page {
  private static $has_many = array(
    'Objects' => 'CommonObject',
  );
}

class CommonObject extends DataObject {
  private static $has_one = array(
    'Parent' => 'DataObject',
  );
}

in simpler words, what I'm trying to achieve, is to have a DataObject that can act as a child in one-to-many relation, having a Page OR a DataObject as a Parent.
But declaring:

  private static $has_one = array(
    'Parent' => 'DataObject',
  );

gives me the following error:
DataList::create Can't find data classes (classes linked to tables) for DataObject
and it is true, as DataObject does not have a separate table in database, as it doesn't have any fields itself.

How do I achieve this?

Many thaks.

Hey, this sounds like you are looking to do polymorphic relations... this is planned for version 3.2 (this is the doc on this, note it only applies to upcoming 3.2 version so is subject to changes http://doc.silverstripe.org/framework/en/trunk/topics/datamodel#has-one) of SilverStripe and in version 3.1 you are likely going to have to you many_many relations and appropriate CMS form components to deal with creating the and assigning CommonObjects to your MyObject and MyPage.