DataProvider::groupBy

private function groupBy($key, $value = true)
{
	if (!$value)
	{
		$pos = array_search($key, $this->group);
		if ($pos !== false)
		{
			unset($this->group[$pos]);
		}
	}
	elseif (!in_array($key, $this->group))
	{
		$this->group[] = $key;
	}

	return $this;
}