Instantiate an object on request or on load using the delegator design pattern

Just looking for varying opinions and reasoning on this:

I want to maintain separate database tables for the delegated class should I:

A) Store the ID of the delegated objects record and initiate it on request like so ...

class Book {

    private $Name;
    private $Color_ID;

    public
    function getColor() {
        return new Color($this - > Color_ID);
    }
    //etc
}

class Color {

    private $Color_ID;
    private $Color_Name;

    public
    function __construct($Color_ID) {
        //Get from DB
    }
    //etc
}

B) Initiate and store the object on the parent initiation:

class B开发者_StackOverflowook {

    private $Name;
    private $Color;

    public
    function __construct() {
        //Get $Color_ID from Books table
        $this - > Color = new Color($Color_ID);
    }

    public
    function getColor() {
        return $Color;
    }
    //etc
}

class Color {

    private $Color_ID;
    private $Color_Name;

    public
    function __construct($Color_ID) {
        //Get from DB
    }
    //etc
}

If I get lots of requests for the color object am I better to store it to save on DB reads? Is increased memory usage a good trade off? Thoughts..

Have a look at the lazy loading pattern.

http://en.wikipedia.org/wiki/Lazy_loading

http://martinfowler.com/eaaCatalog/lazyLoad.html

and probably identity map too:

http://martinfowler.com/eaaCatalog/identityMap.html