First of all, I'd like to say hello to all forum members.
I am a graphic designer and I'm using SilverStripe for small company websites mostly.
I found out that there are many alements repeating in each project, so I'd like to standarize them.
So I decided to use DataObjectDecorators for things like animated slideshows, link groups on Home Page etc, which will be added or not.

I don't know what am I doing wrong, but there are some problems with has_many relation.
I have tho decorators - one is working, the other is not working.
I'm pasting code and dev error.

SlidesHolder with Slider - this is working

<?php

class SlidesHolder extends DataObjectDecorator {

	public function extraStatics(){
		return array(
			'has_many' => array(
				'Slides' => 'Slide'
			)
		);
	}

	public function updateCMSFields(FieldSet &$fields) {
		
		$t_slides = _t('Backend.SLIDES', 'Slideshow');
		
		$fields->addFieldToTab("Root.Content.$t_slides", new HeaderField('Title', 'Title'));
		
		$fields->addFieldToTab("Root.Content.$t_slides", new ImageDataObjectManager(
			$this->owner,
			'Slides',
			'Slide',
			'Photo',
			array(
				'Name'=>'Name'	
			),
			'getCMSFields_forPopup',
			NULL,
			NULL
		));
	}

}

<?php
class Slide extends DataObject {
	
	static $db = array (
	'Name' => 'Varchar(255)',
	'Desc' => 'Text',
	'SortOrder' => 'Int'
	);
	
	static $has_one = array (
	'Photo' => 'Image',
	'PageLink' => 'SiteTree',
	'SlidesHolder' => 'SlidesHolder'
	);
	
	static $default_sort = "SortOrder ASC";  
	
	public function getCMSFields_forPopup() {
		
		return new FieldSet (		
			new TextField('Name', 'Nazwa'),
			new ImageField('Photo', 'Zdjęcie'),
			new TextareaField('Desc', 'Opis'),
			new SimpleTreeDropdownField('PageLinkID', "Link do strony", 'SiteTree'),
			new NumericField ('SortOrder', 'Kolejność', '', 3)			
		);
	}		
	
}


Object::add_extension(SliderConfig::$PageToApply, 'SlidesHolder'); - in _config.php

GraphicLinksHolder with Graphic link - this is not working properly. I can add items by DataObjectManager, but there is error when I try to edit them.
Here's the code:

<?php

class GraphicLinksHolder extends DataObjectDecorator {

	public function extraStatics(){
		return array(
			'db' => array(
			),
			
			'has_many' => array (
				'GraphicLinks' => 'GraphicLink'
			)
		);
	}
    
	public function updateCMSFields(FieldSet &$fields) {
		
		$t_graphiclinks = _t('Backend.GRAPHICLINKS', 'Graphic links');
		$t_links = _t('Backend.LINKS', 'Links');
		
		$fields->addFieldToTab("Root.Content.$t_links", new HeaderField('Title', "$t_graphiclinks"));
		$fields->addFieldToTab("Root.Content.$t_links", new DataObjectManager(
			$this->owner,
			'GraphicLinks',
			'GraphicLink',
			array(
				'Name' => 'Name'
			),
			'getCMSFields_forPopup',
			NULL,
			NULL
		));
	
	}

}


<?php

class GraphicLink extends DataObject
{
	static $db = array (
		'Name' => 'Text',
	);
 
	static $has_one = array (
		'GraphicLinksHolder' => 'GraphicLinksHolder',
		'Photo' => 'Image'
	);
 
	public function getCMSFields_forPopup()
	{
		return new FieldSet(
			new TextField('Name'),
			new ImageField('Photo')
		);
	}
}


Object::add_extension('HomePage', 'GraphicLinksHolder'); - in _Config.php

Error:

[User Error] Couldn't run query: SELECT "GraphicLink"."ClassName", "GraphicLink"."Created", "GraphicLink"."LastEdited", "GraphicLink"."Name", "GraphicLink"."GraphicLinksHolderID", "GraphicLink"."PhotoID", "GraphicLink"."ID", CASE WHEN "GraphicLink"."ClassName" IS NOT NULL THEN "GraphicLink"."ClassName" ELSE 'GraphicLink' END AS "RecordClassName" FROM "GraphicLink" WHERE ("ParentID" = '1') Unknown column 'ParentID' in 'where clause'

I've found it out.
Has_one relation in DataObject needs to point on decorated class, not to decorator itself.