Value vs Entity Objects (Domain Driven Design)

Question

I just started reading DDD. I cannot fully understand the concept of Entity vs Value objects. Can someone explain the problems (maintainability, performance, etc.) that the system may encounter when the Value object is designed as an Entity object? An example will be great ...

Answer 1

Reducing to a significant difference, identity matters to entities, but does not matter to value objects. For example, someone Name is a value object. A client object may consist of the name of the client (value object), the list <Order> OrderHistory (List of objects), and possibly the default address (usually a value object). The customer has an identifier, and each order will have an identifier, but the name should not; as a rule, within the framework of the object model, the identification of the address probably does not matter.

Value objects can usually be represented as immutable objects; changing one property of an object of value significantly destroys the old object and creates a new one, because you are not as concerned about identity as with content. Correctly, the Equals instance method in Name would return "true" if the properties of the object are identical to the properties of another instance.

However, changing any attribute of an object, such as a Client, does not destroy the client; The client object is usually modified. Identity remains unchanged (at least once the object is saved).

You are probably creating value objects without realizing it; anytime you represent any aspect of Entity by creating a fine-grained class, you have a value object. For example, the IPAddress class, which has some restrictions on valid values ​​but consists of simpler data types, will be a value object. EmailAddress can be a string, or it can be a value object with its own set of behavior.

It is possible that even objects that have an identifier in your database do not have an identifier in your object model. But the simplest case is a combination of some attributes that make sense together. You probably don't want to have Customer.FirstName, Customer.LastName, Customer.MiddleInitial and Customer.Title when you can put them together as Customer.Name; they are likely to represent several fields in your database by the time you think about persistence, but your object model does not care.

Answer 2

Any object that is jointly defined by all its attributes is an object of value. If any of the attributes changes, you have a new instance of the value object. This is why value objects are defined as immutable.

If an object is not fully defined by all its attributes, then there is a subset of the attributes that make up the object's identity. Other attributes can change without redefining the object. This type of object cannot be defined in immutable.

The simplest way to make a difference is to think of value objects as static data that will never change, and entities as data that develops in your application.

Answer 3

I do not know if the following is correctly indicated, but I would say that in the case of the Address object, we want to use it as a Value object instead of Entity, because changes in the object will be reflected in all related objects (for example, Person).

Take this case: you live in your house with other people. If we used Entity for Address, I would say that there will be one unique address to which all Person objects will be bound. If one person leaves, you want to update his address. If you update the Address properties, all people will have a different address. In the case of the Value object, we will not be able to edit the address (since it is unchanged), and we will be forced to provide a new address for this person.

Does this sound right? I have to say that I was also confused by this difference by reading the DDD book.

Going further, how will this be modeled in a database? Did you have all the properties of the Address object as columns in the Person table, or would you create a separate address table that would also have a unique identifier? In the latter case, people living in the same house will have another instance of the Address object, but these objects will be the same, except for their identifier properties.

Answer 4

The address can be an entity or value object, which depends on the busiess process. an address object may be an entity in a courier service application, but an address may be a value object in another application. in matters of identification of couriers for an addressee

Answer 5

I asked about it in another thread, and I think I'm still confused. I can confuse performance considerations when modeling data. In our cataloging application, the Client does not change until it is needed. This sounds silly, but “reading” client data far exceeds the number of “records”, and since many “network requests” fall into the “active set” of objects, I don’t want to constantly download Clients. Thus, I set off on the immutable road for the Customer object - downloaded it, cached and served up to 99% of the (multi-threaded) requests that the Client wants to see. Then, when the client changes something, get an “editor” to create a new Client and cancel the old one.

My concern is that many threads see the same client object and change, and then when one thread begins to change, chaos occurs in the rest.

Now my problems are: 1) this is reasonable and 2) the best way to do this without duplicating a lot of property code.

Answer 6

3 difference between Entities and Value Objects

• Identifier and structural equality: Objects have an identifier, entities are the same if they have the same identifier. The value objects outside the hand have structural equality; we consider two value objects equal when all the fields are the same. Value objects cannot have an identifier.

• Mutability and immutability: Value objects are immutable data structures, while objects change during their lifetime.

• Life span: value objects must belong to objects