silverstripe-framework/docs/en/howto/dynamic-default-fields.md

Dynamic Default Values

The [api:DataObject::$defaults] array allows you to specify simple static values to be the default value for when a record is created, but in many situations default values needs to be dynamically calculated. In order to do this, the [api:DataObject->populateDefaults()] method will need to be overloaded.

This method is called whenever a new record is instantiated, and you must be sure to call the method on the parent object!

A simple example is to set a field to the current date and time:

:::php
/**
 * Sets the Date field to the current date.
 */
public function populateDefaults() {
	$this->Date = date('Y-m-d');
	parent::populateDefaults();
}

It's also possible to get the data from any other source, or another object, just by using the usual data retrieval methods. For example:

:::php
/**
 * This method combines the Title of the parent object with the Title of this
 * object in the FullTitle field.
 */
public function populateDefaults() {
	if($parent = $this->Parent()) {
		$this->FullTitle = $parent->Title . ': ' . $this->Title;
	} else {
		$this->FullTitle = $this->Title;
	}
	parent::populateDefaults();
}