It's look simple, but I looked at many posts and I dont found working for me solution. I've got a page with portfolio. This is one page with realisations. Each single realisation subpage is a dataobject. On each one from this subpages I need two buttons: Next and Prev. How to do it?

$prev=$this->Parent()->Children()->filter(['Sort'=>$this->Sort-1]);
//or
$next=SiteTree::get()->filter(['ParentID'=>$this->ParentID,'Sort'=>$this->Sort+1]);

Thanks for your reply. Unfortunately I've got an error with this code: Uncaught Exception: Object->__call(): the method 'parent' does not exist on 'Portfolio'

This pages are not childrens. They are dataobjects like in this tutorial: http://docs.silverstripe.org/en/3.1/tutorials/extending_a_basic_site/

In that case it's harder, without a defined sort.
I believe the default is creation date (which is bascially the ID).
So then

$this->WhateverTheHasOneToTheParentIsNamed()->WhateverTheHasManyToTheseRelatedPortfolioObjectsIsCalled()->filter('Created:GreaterThan',$this->Created)->first()

Or

Portfolio::get()->filter(['ParentRelationNameID'=>$this->ParentRelationNameID, 'Created:LessThan'=>$this->Created])->First()

Where the names of your relations are appropriate (I guess 'PortfolioHolder' maybe?)

Summary page is PortfolioPage. I have errors with your code "unexpected ] ", so I changed it to:

public function PreviousPage() {    
Portfolio::get()->filter(array('PortfolioPageID'=>$this->PortfolioPageID, 'Created:LessThan'=>$this->Created))->First();

}
public function NextPage() {
Portfolio::get()->filter(array('PortfolioPageID'=>$this->PortfolioPageID, 'Created:GreaterThan'=>$this->Created))->First();
  
}

SS is

 <% loop $PreviousPage %>$Title<% end_loop  %>  <% loop $NextPage %>$Title<% end_loop  %>

You need to return those.
return Portfolio::get()->...;

and you should use <% with $NextPage %>, not <% loop %> since you are retrieving a single object, not a list of objects.
Or alternatively $NextPage.Title (etc)

You have right. Thank you for your help.

This almost work for me:

public function PreviousPage() {    
return  $this->PortfolioPage()->Portfolio()->filter('ID:LessThan',$this->ID)->first()  ;
}
//or
public function NextPage() { 
return  $this->PortfolioPage()->Portfolio()->filter('ID:GreaterThan',$this->ID)->first()  ;
}

Next work like charm, but previous show allway firts item with ID =1

Final working code:
PortfolioPage is page
Portfolio extends DataObject {$has_many)

php

public function PreviousPage() {    
return  $this->PortfolioPage()->Portfolio()->filter('ID:LessThan',$this->ID)->sort(array( 'ID' => 'DESC'))->first()  ;
}


public function NextPage() { 
return  $this->PortfolioPage()->Portfolio()->filter('ID:GreaterThan',$this->ID)->sort(array( 'ID' => 'ASC'))->first()  ;
}
 

ss

<% with $PreviousPage %>$Title <% end_with  %>  <% with $NextPage %>$Title <% end_with  %> 

I realy thank you once again. Without you I lost many hours.