Some concepts in our system are more than values.
They are not just cards or small pieces of meaning.
They are… “things” that continue to exist over time.

A user.
A ticket.
An order.
A blog post.

These are not “just data”.
They have a life story.

That's what makes them entities.

Let's return to the desk metaphor.

Imagine a folder labeled “User #17”.

Inside the folder, we keep:

• a name
• an age
• whether the user is active
• maybe a list of posts
• maybe a history of actions

Now imagine that the person changes their name.
Or updates their email. O
r becomes inactive.

We don't throw away the folder.
We update what's inside.

The folder — the identity — stays the same.

That's the essence of an entity:

• An entity is a concept
  whose identity stays the same
  even as its data changes.

Unlike “Value Objects”,
entities are defined by a unique identifier (like a primary key)
rather than just their attributes.

This is where entities differ from value objects.

• A value object is a small piece of meaning.
  If the meaning changes, it becomes a different value.
• An entity is a long‑lived concept.
  If its data changes, it is still the same entity.

A person who changes their name is still the same person.
A price that changes is not the same price.

Identity is the anchor.

Here's a simple entity class:

class User {
    public int $id;
    public string $name;
    public bool $isActive;
}

Let's slow down:

• class User — we're defining a concept
• $id — the unique identifier
• $name — the user's name
• $isActive — whether the user is active

The id is what makes this user this user.
Everything else can evolve.

This is the shape of a long‑lived concept.

$user1 = new User();
$user1->id = 17;
$user1->name = "Tony";
$user1->isActive = true;

Using our metaphor:

• new User() creates a new “User” folder
• $user1 is the label we put on that folder
• $user1→id is the unique identifier inside the folder
• $user1→name and $user1→isActive are the details that can change

The identity is the folder itself.
The contents can be updated.

Let's update the user:

$user1->name = "Anthony";
$user1->isActive = false;

The folder is still “User #17”.
Only the contents changed.

This is the core idea:

Value objects never change.
Entities change all the time — but remain themselves.

Entities often carry rules tied to their identity.

class User {
    public int $id;
    public string $name;
    public bool $isActive;
 
    public function deactivate(): void {
        $this->isActive = false;
    }
}

Let's unpack:

• deactivate() is something the user can do
• $this means “this user”
• $this→isActive = false updates the folder

This rule belongs with the user.
It's part of the concept.

Entities give us:

• identity — a stable reference
• continuity — the concept persists
• behavior — rules tied to the lifecycle
• structure — a clear shape
• safety — fewer invalid states

Entities are the long‑lived concepts in our system.

Use an entity when the concept:

• has identity
• persists over time
• can change
• appears in multiple places
• has rules tied to its lifecycle
• is central to our system

Examples:

• User
• Order
• Invoice
• BlogPost
• Ticket
• Product

These are not “just values”.
They are the living parts of our domain.

Avoid entities when the data is:

• small
• immutable
• just a value
• not meaningful
• not long‑lived

Those belong to value objects.

Entities are long‑lived concepts with identity.
They evolve.
They carry rules.
They tell the story of our system over time.

Use them when:

• the concept continues
• the attributes change
• the identity matters

Entities are the characters in our system's story.

The moment when identity matters

Conversation Example

We ask:

“Create a Post with title and body.”

AI gives us a simple class.

Then we ask:

“Add comments to the post.”

AI gives us:

$post->comments[] = $comment;

Then we ask:

“Allow editing the post later.”

AI adds setters.

Then we ask:

“Store posts in a database.”

AI adds an ID.

Now we feel it:

• this object changes over time
• it has identity
• it has lifecycle
• it has relationships

This is the moment to say:

“This is an Entity. 
Give it an ID and treat it as long‑lived.”

Rule of Thumb

If something changes over time
or has identity — ask for an Entity.

_{Tony de Araujo —New York}

• Top↑
• Back to Topic ↔
• Home←