Law of Demeter - Data objects

Question

I'm trying to follow the Law Of Demeter ( see http://en.wikipedia.org/wiki/Law_of_Demeter , http://misko.hevery.com/code-reviewers-guide/flaw-digging-into-collaborators/ ) as I can see the benefits, however I've become a little stuck when it comes to domain objects.

Domain objects do naturally have a chain and sometimes it's necessary to display the information about the entire chain.

For instance, a shopping basket:

Each order contains a user, delivery info and a list of items Each order item contains a product and quantity Each product has a name and price. Each user contains a name and address

The code which displays the order information has to use all the information about the order, users and products.

Surely it's better and more reusable to get this information through the order object e.g. "order.user.address.city" than for some code higher up to do queries for all the objects I listed above then pass them into the code separately?

Any comments/suggestions/tips are welcome!

Answer 1

The Law Of Demeter is about calling methods, not accessing properties/fields. I know technically properties are methods, but logically they're meant to be data. So, your example of order.user.address.city seems fine to me.

This article is interesting further reading: http://haacked.com/archive/2009/07/13/law-of-demeter-dot-counting.aspx

Answer 2

You're correct and you'll most likely model your value objects something like this

class Order {
    User user;
}

class User {
    Address shippingAddress;
    Address deliveryAddress;
}

class Address {
    String city;
    ...
}

When you start considering how you will persist this data to a database (e.g. ORM) do you start thinking about performance. Think eager vs lazy loading trade offs.

Answer 3

One problem with using chained references, such as order.user.address.city, is that higher-order dependencies get "baked into" the structure of code outside the class.

Ideally, in cases when you refactor your class, your "forced changes" should be limited to the methods of the class being refactored. When you have multiple chained references in the client code, refactoring drives you to make changes in other places of your code.

Consider an example: suppose that you'd like to replace User with an OrderPlacingParty, an abstraction encapsulating users, companies, and electronic agents that can place an order. This refactoring immediately presents multiple problems:

• The User property will be called something else, and it will have a different type
• The new property may not have an address that has city in cases when the order is placed by an electronic agent
• The human User associated with the order (suppose that your system needs one for legal reasons) may be related to the order indirectly, - for example, by being a designated go-to person in the definition of the OrderPlacingParty.

A solution to these problems would be to pass the order presentation logic everything that it needs directly, rather than having it "understand" the structure of the objects passed in. This way you would be able to localize the changes to the code being refactored, without spreading the changes to other code that is potentially stable.

interface OrderPresenter {
    void present(Order order, User user, Address address);
}
interface Address {
    ...
}
class PhysicalAddress implements Address {
    public String getStreetNumber();
    public String getCity();
    public String getState();
    public String getCountry();
}
class ElectronicAddress implements Address {
    public URL getUrl();
}
interface OrderPlacingParty {
    Address getAddress();
}
interface Order {
    OrderPlacingParty getParty();
}
class User implements OrderPlacingParty {
}
class Company implements OrderPlacingParty {
    public User getResponsibleUser();
}
class ElectronicAgent implements OrderPlacingParty {
    public User getResponsibleUser();
}

Answer 4

I think, when chaining is used to access some property, it is done in two (or at least two) different situation. One is the case that you have mentioned, for example, in your presentation module, you have an Order object and you would like to just display the owner's/user's address, or details like city. In that case, I think it is of not much problem if you do so. Why? Because you are not performing any business logic on the accessed property, which can (potentially) cause tight coupling.

But, things are different if you use such chaining for the purpose of performing some logic on the accessed property. For example, if you have,

String city = order.user.address.city;
...
order.user.address.city = "New York";

This is problematic. Because, this logic is/should more appropriately be performed in a module closer to the target attribute - city. Like, in a place where the Address object is constructed in the first place, or if not that, at least when the User object is constructed (if say User is the entity and address the value type). But, if it goes farther than that, the farther it goes, the more illogical and problematic it becomes. Because there are too many intermediaries are involved between the source and the target.

Thus, according to the the Law of Demeter, if you are performing some logic on the "city" attribute in a class, say OrderAssmebler, which accesses the city attribute in a chain like order.user.address.city, then you should think of moving this logic to a place/module closer to the target.

Answer 5

Generally speaking I adhere to the Law of Demeter since it helps to keep changes in a reduced scope, so that a new requirement or a bug fix doesn't spread all over your system. There are other design guidelines that help in this direction, e.g. the ones listed in this article. Having said that, I consider the Law of Demeter (as well as Design Patterns and other similar stuff) as helpful design guidelines that have their trade-offs and that you can break them if you judge it is ok to do so. For example I generally don't test private methods, mainly because it creates fragile tests. However, in some very particular cases I did test an object private method because I considered it to be very important in my app, knowing that that particular test will be subject to changes if the implementation of the object changed. Of course in those cases you have to be extra careful and leave more documentation for other developers explaining why you are doing that. But, in the end, you have to use your good judgement :).

Now, back to the original question. As far as I understand your problem here is writing the (web?) GUI for an object that is the root of a graph of objects that can be accessed through message chains. For that case I would modularize the GUI in a similar way that you created your model, by assigning a view component for each object of your model. As a result you would have classes like OrderView, AddressView, etc that know how to create the HTML for their respective models. You can then compose those views to create your final layout, either by delegating the responsibility to them (e.g. the OrderView creates the AddressView) or by having a Mediator that takes care of composing them and linking them to your model. As an example of the first approach you could have something like this (I'll use PHP for the example, I don't know which language you are using):

class ShoppingBasket
{
  protected $orders;
  protected $id;

  public function getOrders(){...}
  public function getId(){...}
}

class Order
{
  protected $user;

  public function getUser(){...}
}

class User
{
  protected $address;

  public function getAddress(){...}
}

and then the views:

class ShoppingBasketView
{
  protected $basket;
  protected $orderViews;

  public function __construct($basket)
  {
     $this->basket = $basket;
     $this->orederViews = array();
     foreach ($basket->getOrders() as $order)
     {
        $this->orederViews[] = new OrderView($order);
     }
  }

  public function render()
  {
     $contents = $this->renderBasketDetails();
     $contents .= $this->renderOrders();     
     return $contents;
  }

  protected function renderBasketDetails()
  {
     //Return the HTML representing the basket details
     return '<H1>Shopping basket (id=' . $this->basket->getId() .')</H1>';
  }

  protected function renderOrders()
  {
     $contents = '<div id="orders">';
     foreach ($this->orderViews as $orderView)
     {
        $contents .= orderViews->render();
     }
     $contents .= '</div>';
     return $contents;
  }
}

class OrderView
{
//The same basic pattern; store your domain model object
//and create the related sub-views

  public function render()
  {
     $contents = $this->renderOrderDetails();
     $contents .= $this->renderSubViews();
     return $contents;
  }

  protected function renderOrderDetails()
  {
     //Return the HTML representing the order details
  }

  protected function renderOrders()
  {
     //Return the HTML representing the subviews by
     //forwarding the render() message
  }
}

and in your view.php you would do something like:

$basket = //Get the basket based on the session credentials
$view = new ShoppingBasketView($basket);
echo $view->render();

This approach is based on a component model, where the views are treated as composable components. In this schema you respect the object's boundaries and each view has a single responsibility.

Edit (Added based on the OP comment)

I'll assume that there is no way of organizing the views in subviews and that you need to render the basket id, order date and user name in a single line. As I said in the comment, for that case I would make sure that the "bad" access is performed in a single, well documented place, leaving the view unaware of this.

class MixedView
{
  protected $basketId;
  protected $orderDate;
  protected $userName;

  public function __construct($basketId, $orderDate, $userName)
  {
    //Set internal state
  }


  public function render()
  {
    return '<H2>' . $this->userName . "'s basket (" . $this->basketId . ")<H2> " .
           '<p>Last order placed on: ' . $this->orderDate. '</p>';
  }
}

class ViewBuilder
{
  protected $basket;

  public function __construct($basket)
  {
    $this->basket = $basket;
  }

  public function getView()
  {
     $basketId = $this->basket->getID();
     $orderDate = $this->basket->getLastOrder()->getDate();
     $userName = $this->basket->getUser()->getName();
     return new MixedView($basketId, $orderDate, $userName);
  }
}

If later on you rearrange your domain model and your ShoppingBasket class can't implement the getUser() message anymore then you will have to change a single point in your application, avoid having that change spread all over your system.

HTH