gistfile1.php

public function Availability() {
  $check = $this->Places - $this->Bookings()->Sum('NumSlots');
		if ($check < 1) {
			return "Unavailable";
		} else {
			return "Available";
		}
}

public function getAvailablePlaces() {
    return $this->Places - $this->Bookings()->Sum('NumSlots');
}

public function getAvailablePlacesDropDown() {
  	$placesremaining = $this->Places - $this->Bookings()->Sum('NumSlots');
		$range = range(1, $placesremaining, 1);
		$fieldset = new DropdownField('NumSlots', 'NumSlots', array_combine($range, $range));
		return $fieldset;
	
}

public $AvailablePlaces;

public function init() {
        parent::init();
        $this->AvailablePlaces = $this->Places - $this->Bookings()->Sum('NumSlots');
}

public function Availability() {
        if ($this->AvailablePlaces < 1) {
            return "Unavailable";
        } else {
            return "Available";
        }
}

public function getAvailablePlaces() {
    return $this->AvailablePlaces;
}

public function getAvailablePlacesDropDown() {
        $range = range(1, $this->AvailablePlaces, 1);
        $fieldset = new DropdownField('NumSlots', 'NumSlots', array_combine($range, $range));
        return $fieldset;

}