Domain Modeling Entities

Entities

Objects that have a distinct identity that runs through time and different representations.
-- Eric Evans

An Entity is a fundamental concept representing a distinct, identifiable object with a continuous and evolving lifecycle. Unlike Value Objects, which are defined by their attributes and are interchangeable based on their values, Entities are distinguished by a unique identity that persists over time.

Getting Started

Creating an Entity with the Complex Heart Domain Model is really simple. The library provides a trait named IsEntity that, when used, adds four essential traits to your class:

• HasAttributes: Allows the Entities to have attributes.
• HasInvariants: Enables the definition and validation of invariants.
• HasEquality: Facilitates the implementation of equality comparison.
• HasIdentity: Provides identification by unique id.

Additionally, you can implement the Entity interface to expose the values and equals methods, providing a consistent interface for all Entities.

Example

Let's illustrate the implementation of a Customer Entity using the provided traits and interface:

final class Customer implements Entity
{
    use IsEntity;

    public function __construct(
        public readonly UUIDValue $id,
        public string $name,
    ) {
        $this->check();
    }

    public function id(): Identifier
    {
        return $this->id;
    }

    public function __toString(): string
    {
        return "$this->name ($this->id)";
    }
}

Key Concepts

Identity

Entities have a distinct identity that sets them apart from other objects. This identity often persists across different states or versions of the object.

Lifecycle

Entities have a lifecycle that spans various states or transitions. They exist beyond a single operation or transaction, maintaining continuity and relevance over time.

Mutability

Unlike Value Objects, Entities can undergo changes in their internal state while retaining the same identity. This mutability is often a crucial aspect of their behavior.

Equality Based on Identity

Equality for Entities is based on identity rather than the equality of attribute values. Two entities with the same identity are considered equal, regardless of potential differences in other attributes. You can use the equals method with another entity, in this case the id will be used.

$c1 = new Customer(UUIDValue::random(), 'Vincent Vega');
$c2 = new Customer(UUIDValue::random(), 'Marcellus Wallace');
$c1->equals($c2); // returns false

$c2->name = 'Vincent Vega';
$c1->equals($c2); // returns false
$c1->equals($c1); // return true

Consistency and Business Rules

Entities encapsulate and enforce business rules and logic related to their specific domain. They are responsible for maintaining consistency within their boundaries. You can use invariants the same way that with the Value Objects.

protected function invariantSomeInvariantForCustomer(): bool 
{
    // do some validations.
}