Domain Entity

Domain Entity

Goal

Define a domain entity as a domain object with a unique identity that persists over time, even as its attributes change.

A domain entity encapsulates business behavior, business rules, and invariants related to its own data. It acts as a core building block in the domain or business layer for concepts such as a customer, order, subscription, account, or shipment.

Treat a domain entity as identity-driven code. The key question is whether the object represents a specific domain concept that remains the same thing throughout its lifecycle while its state evolves.

What Counts as a Domain Entity

Classify code as a domain entity when it does one or more of these things:

• represents a specific domain concept through a stable identity
• carries an identifier that distinguishes one instance from another across time
• preserves continuity as state changes throughout a lifecycle
• exposes behaviors that apply domain rules to its own data
• enforces invariants that must remain true for that specific domain concept
• controls valid state transitions for its own lifecycle
• encapsulates internal state and requires callers to use intention-revealing methods
• protects its data from arbitrary mutation so validity is maintained

Domain entities often appear in code that answers questions such as:

• which specific customer, order, account, or shipment is this
• what makes this object the same domain thing after its attributes change
• which operations may change its state
• which conditions must remain true throughout its lifecycle
• how callers must interact with it to keep it valid

Detection Workflow

1. Read the code for identity first.

   • Look for identifiers, natural keys, references, or equality rules that distinguish one instance from another.
   • Pay attention to code that tracks the same domain concept over time rather than only its current attributes.

2. Identify lifecycle continuity.

   • Determine whether the object remains the same domain concept as its properties change.
   • Identify the states, transitions, and milestones that define its lifecycle.

3. Trace behaviors and invariants.

   • Identify methods that change the entity's state, enforce rules, or reject invalid transitions.
   • Identify the conditions that must always hold true for the entity to remain valid.

4. Check the encapsulation boundary.

   • Identify whether callers are expected to change state through explicit methods instead of direct arbitrary mutation.
   • Prefer designs where domain operations are expressed as named behaviors with domain meaning.

5. Prefer semantic classification to file or framework conventions.

   • Do not assume code is or is not a domain entity only because of its folder, class name, annotation, ORM mapping, or framework role.
   • Classify by whether the object models a stable domain identity with owned behavior and invariants.

Writing or Changing Domain Entities

1. Preserve the identity model before refactoring.

   • Restate what makes the entity the same domain concept over time.
   • Keep identifiers and identity semantics explicit.

2. Use a class to co-locate identity, state, invariants, and behavior.

   • Use a class, or the closest class-like construct available in the language, so that identity, state, invariants, and domain behavior live together in a single construct.
   • A plain type alias, interface, or record paired with standalone functions is not a class-like construct. Do not model a domain entity that way.
   • Follow the project's existing conventions for how classes are modeled.
   • Only fall back to a non-class construct when the language has no class support at all, not merely because a functional style is common or preferred by convention.

3. Keep behavior close to the entity.

   • Put state-changing domain operations on the entity when they belong to that entity's own rules.
   • Use method names that express business intent, such as changeAddress, approve, cancel, or assignOwner.

4. Protect invariants through explicit operations.

   • Verify that every allowed state change still enforces the required rules.
   • Avoid exposing write paths that let callers bypass validity checks.

5. Preserve lifecycle integrity.

   • Verify that transitions between states remain valid and understandable.
   • Verify that the entity keeps the same identity across creation, loading, updates, and later lifecycle stages.

6. Hide internal structure where validity depends on it.

   • Expose the minimum data and methods needed for callers to interact correctly.
   • Prefer intention-revealing methods over unrestricted setters when rules must be enforced.

Review Questions

When reading or reviewing code, ask:

• What makes this object the same domain thing over time?
• Which identifier distinguishes it from other instances?
• Which behaviors belong to this object as part of its own domain responsibility?
• Which invariants must remain true for it to stay valid?
• Would changing this code alter the identity, lifecycle, or rule enforcement of a specific domain concept?

If the answer is yes, treat the code as a domain entity.

Report the Outcome

When finishing the task:

• state which code was identified or treated as a domain entity
• state whether a class or equivalent construct was used, and why
• state which identities, behaviors, lifecycle transitions, or invariants were implemented or preserved
• state which methods or rules protect the entity's validity