实体引用与 @key 指令
掌握 Apollo Federation 的核心机制：了解子图如何使用 @key 指令共享实体，以及如何在 Spring Boot 中解析来自其他子图的引用。
实体引用与 @key 指令 是 CoddyKit 上的免费 GraphQL APIs with Spring Boot 课时。 这是第 4 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 GraphQL APIs with Spring Boot 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 GraphQL APIs with Spring Boot 课程共包含 4 节课。
本课时的部分内容尚未翻译，以英文显示。
Entities Span Subgraphs
In federation, a single type like Product can be owned and extended by several subgraphs. The mechanism that lets them agree on the same object is the entity.
What Makes an Entity
An entity is a type with a @key directive. The key names the field(s) that uniquely identify an instance across subgraphs, like a primary key in a database.
type Product @key(fields: "id") {
id: ID!
name: String!
}The Owning Subgraph
The subgraph that defines a type's core fields is its owner. It is responsible for fetching a full entity given just its key, so the gateway can stitch data together.
Extending an Entity Elsewhere
Another subgraph can add fields to the same entity. It declares the type with the matching @key and marks borrowed fields as external context.
type Product @key(fields: "id") {
id: ID!
reviews: [Review!]!
}The Reference Resolver
When the gateway needs an entity from a subgraph, it calls a special reference resolver, passing only the key fields. The subgraph returns the matching object.
Resolving References in Spring
Spring for GraphQL Federation provides @EntityMapping to implement the reference resolver. The key fields arrive as arguments.
@EntityMapping
public Product product(@Argument String id) {
return productService.findById(id);
}Adding the Federation Library
Enable federation by adding the Apollo federation JVM support, which generates the _entities and _service fields the gateway requires.
// build.gradle
implementation 'com.apollographql.federation:federation-graphql-java-support'Compound Keys
An entity can be identified by multiple fields. List them space-separated in the @key, useful when no single field is unique.
type OrderItem @key(fields: "orderId sku") {
orderId: ID!
sku: String!
}Multiple Keys
A type can declare several @key directives, letting different subgraphs reference it by whichever identifier they hold.
type User @key(fields: "id") @key(fields: "email") {
id: ID!
email: String!
}How the Gateway Stitches Data
The gateway queries the owning subgraph, then sends the entity's keys to other subgraphs via _entities to fetch their extra fields, merging everything into one response.
Best Practices
Design entities carefully:
- Pick stable, unique key fields
- One subgraph owns the core fields
- Keep reference resolvers fast (consider DataLoaders)
- Use compound keys only when necessary
Quick Check
Test your federation entity knowledge.
Recap
You learned federated entities:
- An entity is a type with a
@key - One subgraph owns it; others extend it
- Reference resolvers (
@EntityMapping) fetch by key - Compound and multiple keys handle complex identity
Entities and @key are how federation stitches a unified graph from many subgraphs.
常见问题解答
「实体引用与 @key 指令」课时是免费的吗？
是的 — 「实体引用与 @key 指令」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 GraphQL APIs with Spring Boot 课程的其余内容，请升级到 CoddyKit PRO。 GraphQL APIs with Spring Boot 课程共包含 4 节课。
「实体引用与 @key 指令」这节课中我会学到什么？
掌握 Apollo Federation 的核心机制：了解子图如何使用 @key 指令共享实体，以及如何在 Spring Boot 中解析来自其他子图的引用。 你通过在浏览器中直接运行的动手代码来练习 GraphQL APIs with Spring Boot，全天候 AI 导师会在你学习这节课的过程中回答你的问题。
学习 GraphQL APIs with Spring Boot 需要有经验吗？
无需任何先前经验。CoddyKit 上的 GraphQL APIs with Spring Boot 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 4 节课，共 4 节。
「实体引用与 @key 指令」课时需要多长时间？
大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。
我能在这节 GraphQL APIs with Spring Boot 课中编写并运行代码吗？
能。每节 GraphQL APIs with Spring Boot 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。
此课程中的所有课时
- Apollo Federation 入门
- 构建联邦子图
- 网关设置与管理
- 实体引用与 @key 指令