populateDefaults in extension

<%- if @topic_view.topic.tags.present? %>
<%= t 'js.tagging.tags' %>: <%- @topic_view.topic.tags.each do |t| %> <%= t %> <%- end %>
<% end %>

Hi I’m using SS 4.6

I’m extending DataObjects with DataExtensions.
In the DataExtension I want to use populatDefaults to set some default-values… The function is called, but no values are safed.
Any idea?

public function populateDefaults() 
	{
		$routes=array();
		foreach(Route::get()->filter('IsSpecial','0') as $r){
			array_push($routes,$r->ID);
		}
		$routes=implode(",",$routes);
		$this->Routes =$routes;
		parent::populateDefaults();
	}

thanks in advance
fabi