Creating a boolean form widget that accepts null value - symfony1

Question

My Symfony model has a field which is boolean but also accepts NULL so effectively is a tri-state value.
How can I write a widget for this? Symfony auto-generates a sfWidgetFormCheckbox but that
can not be set to NULL.
I tried a sfWidgetFormChoice with but I had to specify the values as strings to get them work:
$this->setWidget('wt', new sfWidgetFormChoice(array(
'choices' => array(true => 'true', false => 'false', null => 'null')
)));
It works for storing values but whenever I save a "false" value, the select jumps back to 'null'. I tried several combinations of 'false', '0', '' etc. but got nothing to work in all three cases.
Any ideas?

Answer 1

An example right from the docs:
class sfWidgetFormTrilean extends sfWidgetForm
{
public function configure($options = array(), $attributes = array())
{
$this->addOption('choices', array(
0 => 'No',
1 => 'Yes',
'null' => 'Null'
));
}
public function render($name, $value = null, $attributes = array(), $errors = array())
{
$value = $value === null ? 'null' : $value;
$options = array();
foreach ($this->getOption('choices') as $key => $option)
{
$attributes = array('value' => self::escapeOnce($key));
if ($key == $value)
{
$attributes['selected'] = 'selected';
}
$options[] = $this->renderContentTag(
'option',
self::escapeOnce($option),
$attributes
);
}
return $this->renderContentTag(
'select',
"\n".implode("\n", $options)."\n",
array_merge(array('name' => $name), $attributes
));
}
}