core-bundle/src/Resources/contao/library/Contao/Model.php
public function onUnregister(Registry $registry)
	{
		// Unregister aliases to unique fields
		foreach (static::getUniqueFields() as $strColumn)
		{
			$varAliasValue = $this->{$strColumn};

			if ($registry->isRegisteredAlias($this, $strColumn, $varAliasValue))
			{
				$registry->unregisterAlias($this, $strColumn, $varAliasValue);
			}
		}
	}
public function onUnregister(Registry $registry)
	{
		// Unregister aliases to unique fields
		foreach (static::getUniqueFields() as $strColumn)
		{
			$varAliasValue = $this->{$strColumn};

			if ($registry->isRegisteredAlias($this, $strColumn, $varAliasValue))
			{
				$registry->unregisterAlias($this, $strColumn, $varAliasValue);
			}
		}
	}
Called when the model is detached from the model registry

@param Registry $registry The model registry