silverstripe-framework/docs/en/02_Developer_Guides/00_Model/How_Tos/Dynamic_Default_Fields.md

Dynamic Default Values

The DataObject::$defaults array allows you to specify simple static values to be the default values when a record is created, but in many situations default values need to be dynamically calculated. In order to do this, the 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:

/**
 * 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:

/**
 * 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();
}