DDD - Entity vs ValueObject

Question

I was reading about DDD and I realize that sometimes an entity might be a VO or VO might be an entity. You can know which one is better depends on the context. I was checking different examples. For example, shopping cart DDD example. Product is an aggregate root, Cart is an aggregate root and items is a entity of Cart, so if you want to add a product to the cart you would do something like this:

$cart->addProduct(id $id, $name, $price)

class Cart
{
    private items;
    function addProduct(ProductId $id, ProductName $name, ProductPrice $price) {
        this->items[] = new Item(
            new ItemProductId($id->ToString()),
            new ItemName($name->ToString()),
            new ItemPrice($price->ToString()),
            new ItemCartId(this->id->ToString())
        );
    }
}

There are two reasons why I think it is a VO:

1. You cannot modify the value's item ( only if the product's price has been modify there is a event that would modify its price).
2. The item doesn't have id, it's has a reference of the product(ItemProductId) and a reference of the cart (ItemCartId)

Answer 1

I was reading about DDD and I realize that sometimes an entity might be a VO or VO might be an entity. You can know which one is better depends on the context.

Usually its pretty clear whats entity and whats an value object. If it contains data that's fixed at the time of assignation, its a value object. For example "order address" on the order aggregate. When the order is placed, the address is set. "Addresses" may be an entity in user aggregate (i.e. a list of his common addresses), but for an order its an value object since its not supposed to change when the user edits or deletes one of his addresses.

cart->addProduct(id $id, $name, $price)

class Cart
{
    private items;
    function addProduct(ProductId $id, ProductName $name, ProductPrice $price) {
        this->items[] = new Item(
            new ItemProductId($id->ToString()),
            new ItemName($name->ToString()),
            new ItemPrice($price->ToString()),
            new ItemCartId(this->id->ToString())
        );
    }
}

That's a pretty bad example. Why would or should the value object be ItemPrice? Does that makes it any special? Why string? A price is usually just a numeric value but also involves a currency, passing it as string kinda beats that.

On top of that, having ItemCartId in the it does a) leak data persistence knowledge into your domain. The fact, it's contained inside this->items[] already establishes a relationship between the entity (or aggregate) and the value object. ItemCartId as no meaning in the domain, other than that it's required for relational database engines (=persistence knowledge)

There are two reasons why I think it is a VO:

You cannot modify the value's item ( only if the product's price has been modify there is a event that would modify its price).

You sure? Why would a eCommerce business want to have the prices in the card anyways?

Prices are informational only, they could change before the order is placed. Same as availability.

A lot of users put stuff in their cart and check on next day. In that time, the price could change.

No company would want to sell a product for the price when it was put into the shopping cart, if the price increased in the time since it was put in there. That would mean a financial loss.

Prices in the shopping carts are informational, not compulsory. You need know the exact process of the company.

The item doesn't have id, it's has a reference of the product(ItemProductId) and a reference of the cart (ItemCartId) Again. Why do you think ItemCartId belongs to the Item object? That's leaked persistence knowledge, since its only important for relational database systems.

All you really need in a shopping cart is * product or article number (not necessary the id, that's typically db knowledge) * quantity

Nothing else. If you may want to change the user when the price changed and show the old and new price, the take the price (=currency value object, not ItemPrice) to it too as a value to compare to an old state.

Finally and probably most importantly: Consider if the shopping cart is an aggregate at all (or does fit into ddd).

After all, most shopping carts are only a value bag w/o a lot of business logic into it. The real logic (checking the real price, product availability, asking for shipping location, calculation of taxes and shipping costs) happens during the checkout process, not while putting stuff into the cart.

For example you can check out eShops on Containers demo project showing an example shopping service based on microservices and ddd.

Some microservices apply DDD (such as Ordering microservice), where others don't (Catalog microservice or the Basket (cart) Microservice).

Applying DDD doesn't mean everything needs to be done with DDD. If its simple crud based services, then you don't need DDD for these. DDD adds a value where you have complex systems and complex business logic.

A catalog has neither, it just presents data which come from a different system (i.e. ERP which on other side may be built on using DDD).

Answer 2

I don't understand what are you asking exactly, but the code you are providing could be improved.

First of all I suggest you to read the red book by Vaughn Vernon https://www.amazon.co.uk/Implementing-Domain-Driven-Design-Vaughn-Vernon/dp/0321834577: you can find 3 chapters describing how to define entities, value objects and aggregates, with some rules of thumbs.

One of those advices, is to keep your aggregates as small as possible, in order to improve your performance and keep the code easy to read and maintain. Imagine that you have a Blog aggregate that contains a list of Post entities: if you manage all of them in a single aggregate, when you want to modify the blog Author, for example, you are forced to retrieve all of the blog's post, without any reason, and that means that you are doing a join and slowing down your application. The more your aggregates grows, the slower those queries with their joins.

So, in the case of the Cart, I suggest you to build the cart without any item or product, instead you can add the CartId to the Item. Cart does not know which items it contains, but items know in which cart they are.

About value objects: is a tool that allows you to wrap some validation and business logic inside a class that is represented by its state and not by its id (like entities), so in the case of the cart, if you put two identical bottles of water inside it, how can you know that they are different? Do you need to know that they are different? Are they different if they are physically (or logically different) or are they different if some of their attribute is different?

In my opinion an item or a product, in your case, are entities because they are not measuring anything, and when you put an item twice, you actually have two different items (and you use an id to recognize them).

This is not necessary like this, sometime you can use a value object and sometimes an entity, it depends on your context. A good example to understand that difference is with money:

1. if you want to measure an amount, for example 10 dollars, probably a value object will work for you, because you don't care if it a bill or another, you just want to measure 10 dollars; in this case if you have 10 dollars, is not important if you have a bill or another, the important thing is that is 10 and not 5

2. in the case that you need to recognize different physical bills, for any reason (you need to track money for the police), you should use an entity, because any printed bill has a unique serial number, and a 10 dollar bill, in this context, is actually different from another 10 dollar bill

Hope this can help you.

Goodbye!