ddd-entity

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🇺🇸

Original

English

🇨🇳

Translation

Chinese

One-line summary

一句话总结

An entity is a domain object identified by who it is, not by what it contains — its identity is stable across attribute changes and across time.

Entity 是由 who it is（它是谁）而不是 what it contains（它包含什么）来标识的 domain object——其 identity 在 attributes 发生变化以及时间推移时都保持稳定。

When to use this skill

何时使用此 skill

Modeling something tracked over time:
```
Customer
```
,
```
Order
```
,
```
Account
```
,
```
Shipment
```
,
```
Subscription
```
.
Two instances with identical attributes are still distinct things ("two customers named John Smith are not the same customer").
A concept has a lifecycle — created, modified, archived, deleted — and the same conceptual instance persists across those states.

对需要随时间跟踪的对象建模：
```
Customer
```
、
```
Order
```
、
```
Account
```
、
```
Shipment
```
、
```
Subscription
```
。
两个实例即使具有完全相同的 attributes，仍然是不同的对象（“两个名为 John Smith 的 Customer 不是同一个 Customer”）。
某个 concept 具有 lifecycle——创建、修改、归档、删除——并且同一个 conceptual instance 会持续存在于这些状态之中。

When NOT to use this skill

何时不要使用此 skill

The concept is fully described by its attributes with no identity → use
```
ddd-value-object
```
.
The "identity" is just a database autoincrement with no domain meaning → that suggests the concept might actually be a value object that the persistence layer is identifying for storage reasons.
The concept exists only inside a single aggregate and is never referenced from outside → it may still be an entity, but reconsider whether it needs to be modeled separately at all.

该 concept 完全由其 attributes 描述且没有 identity → 使用
```
ddd-value-object
```
。
所谓的“identity”只是 database autoincrement ID，而不具有 domain 意义 → 这可能意味着该 concept 实际上是一个 Value Object，只是 persistence layer 为了存储而将其标识出来。
该 concept 仅存在于单个 aggregate 内部，并且从不被外部引用 → 它仍然可能是 Entity，但应重新考虑是否有必要将其单独建模。

Core content

核心内容

Identity is the distinguishing feature. An entity's identity is assigned (often as a UUID, a business identifier like a customer number, or a natural key) and stable — it does not change when attributes change. Equality is by identity: two entities are equal if and only if their identifiers match, regardless of every other field.

Entities have lifecycles. They are created (often via a factory), modified through methods that maintain invariants, possibly archived, and possibly deleted. Throughout that lifecycle the identity remains the same.

Entities expose behavior, not just data. A

Customer

entity might have methods like

changeAddress

closeAccount

deactivate

. Each of those methods is responsible for enforcing the entity's invariants — the customer's state is never inconsistent.

Identity 是区分 Entity 的核心特征。Entity 的 identity 是被 assigned 的（通常采用 UUID、customer number 之类的 business identifier，或者 natural key），并且保持 stable——当 attributes 发生变化时，它不会改变。相等性由 identity 决定：无论其他所有字段如何，两个 Entity 当且仅当其 identifiers 匹配时才相等。

Entity 拥有 lifecycle。它们被创建（通常通过 factory），通过维护 invariants 的方法进行修改，可能被 archive，也可能被 delete。在整个 lifecycle 中，identity 始终保持不变。

Entity 暴露的是 behavior，而不只是 data。例如，

Customer

Entity 可能包含

changeAddress

、

closeAccount

、

deactivate

等方法。每个方法都负责维护该 Entity 的 invariants——Customer 的状态永远不会处于不一致状态。

Decision heuristics

决策启发式

Entity if you need to track its history or refer to "the same one" later.
Value object if you would happily swap one instance for another with the same attributes.
If a candidate concept is an entity but has no behavior beyond getters/setters, push back: either the entity is anemic (add behavior), or it is misclassified (it might really be a value object).
Identity is assigned, not derived from attributes. If you find yourself deriving identity from fields, you are likely modeling a value object.

如果需要追踪对象的 history，或者之后需要引用“同一个”对象，则应使用 Entity。
如果可以将一个实例替换为另一个具有相同 attributes 的实例而不会产生问题，则应使用 Value Object。
如果一个 candidate concept 是 Entity，但除了 getters/setters 之外没有任何 behavior，则应提出异议：要么该 Entity 是 anemic 的（需要补充 behavior），要么它分类错误（实际上可能是 Value Object）。
Identity 是被 assigned 的，而不是从 attributes 推导出来的。如果发现通过 fields 推导 identity，那么所建模的对象很可能实际上是 Value Object。

Anti-patterns

反模式

Modeling every domain object as an entity, including ones that have no inherent identity ("entity explosion").
Entity with no behavior — an "anemic entity" that is just fields with getters. See
```
ddd-anti-patterns
```
.
Replacing identity equality with attribute equality (a language adapter pitfall — Kotlin
```
data class
```
does this by default).
Treating a database row's autoincrement ID as if it were the domain's identity.

将每个 domain object 都建模为 Entity，包括那些没有 inherent identity 的对象（“entity explosion”）。
没有 behavior 的 Entity——即只有 fields 和 getters 的“anemic entity”。参见
```
ddd-anti-patterns
```
。
使用 attribute equality 代替 identity equality（language adapter pitfall——Kotlin
```
data class
```
默认采用前一种方式）。
将 database row 的 autoincrement ID 当作 domain 的 identity。

References

参考资料

Evans, Domain-Driven Design (2003), Ch. 5 (A Model Expressed in Software).

Evans，Domain-Driven Design（2003），第 5 章（A Model Expressed in Software）。