在创建对象之前需要调用辅助函数时，我使用什么设计模式？

Question

I have a class Motor , which I use to provide me with various motor specifications.

I currently have the following methods as part of the Motor class:

public function getIdFromModel($model)
{
    $sql = "SELECT id FROM ... WHERE model = ...";
    return process($sql);
}

public function getIdByFrame($size)
{
    $sql = "SELECT id FROM ... WHERE size = ...";
    return process($sql);
}

The above are supplementary functions used to get motor's id from database, when some other non- id parameter is known. id is then used to create the actual motor.

Before I could do:

$model = "ABC";
$motor = new Motor();

//retrieve ID
$id = $motor->getIdFromModel($model)

$motor-loadSpecs($id);
$motor = new Motor($id);

My problem shows up after I have changed the structure of my class to require knowledge of id at Motor creation time. Now I need to do:

$model = "ABC";

//retrieve ID
$id = $???->getIdFromModel($model)

$motor = new Motor($id);

Question

What structure/class/pattern/function do I use in place of ??? above?

I know that I can create a class just for the above two functions, but I feel there is a better answer/pattern for this and I wanted to ask. I don't see a meaning for creating a class just for something-to-id methods. I'd like to have some meaning.

Answer 1

You would normally use a Factory Patter for this. It could be a stand-alone function or a static class method depending on how you want to implement it.

http://www.oodesign.com/factory-pattern.html

Answer 2

I'd make them static functions, member of Motor.

public static function getIdFromModel($model)
{
  ..
}

Then you can call the function, without the need of an instance.

$id = Motor::getIdFromModel($model)

Answer 3

Factory Method + supporting methods

Usage:

$id = 17;
$model = "ABC";
$frame = "T123";

$factory = new MotorFactory();

//instantiation via various pieces of data
$motor = $factory->fromId($id);
$motor = $factory->fromModel($model);
$motor = $factory->fromFrame($frame);

Factory Method:

class MotorFactory
{
    public function fromId($id)
    {
        return new Motor($id);
    }

    public function fromModel($model)
    {
        $id = $this->getIdFromModel($model);
        return $this->fromId($id);
    }

    public function fromFrame($frame)
    {
        $id = $this->getIdFromFrame($frame);
        return $this->fromId($id);
    }

    //supporting methods
    private function getIdFromModel($model)
    {
        $sql = "SELECT id FROM ... WHERE model = ...";
        return process($sql);
    }

    private function getIdFromFrame($size)
    {
        $sql = "SELECT id FROM ... WHERE size = ...";
        return process($sql);
    }
}