练习Practice
动手做Get your hands on it
练习跟着课文走 —— 每节课尾都有本课的练习。这一页是全部练习的总库，想集中刷题的时候来。 每个练习都写清了它来自哪一节，卡住了就回去看那一节。Practice follows the lessons — every lesson ends with the exercises for that lesson. This page is the whole library, for when you want to drill in one sitting. Each exercise names the lesson it came from, so you can go back when you stall.
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练习Exercises
筛出 3 个练习（共 148 个）。Showing 3 of 148.来自From N+1 问题与 DataLoaderThe N+1 problem and DataLoader · Federation 考试Federation exam
L2Debug LabDebug LabDebug Lab · DataLoader 报 is not a functionDebug Lab · DataLoader reports is not a function
跑 npm test，其中一个 DataLoader 相关的测试挂了。 报错指向 loader 内部。
You run npm test and one of the DataLoader tests fails. The error points inside the loader.
● Order Resolvers › DataLoader functionality › should batch multiple order requests
TypeError: orderDataSource.getOrderById is not a function
29 |
30 | const orders = await Promise.all(
> 31 | orderIds.map(id => orderDataSource.getOrderById(id))
| ^
32 | );
33 |
34 | return orders;
at src/resolvers/orderResolvers.js:31:42
at Array.map (<anonymous>)
at DataLoader._batchLoadFn (src/resolvers/orderResolvers.js:31:16)
1function createOrderLoader(orderDataSource) {
2 return new DataLoader(async orderIds => {
3 const orders = await Promise.all(
4 orderIds.map(id => orderDataSource.getOrderById(id))
5 );
6 return orders;
7 });
8}
9
10// 参考：OrderDataSource 上真实存在的方法
11// class OrderDataSource {
12// async getOrder(id) { ... }
13// async getOrdersByUserId(userId) { ... }
14// async createOrder(userId, items) { ... }
15// }
1function createOrderLoader(orderDataSource) {
2 return new DataLoader(async orderIds => {
3 const orders = await Promise.all(
4 orderIds.map(id => orderDataSource.getOrderById(id))
5 );
6 return orders;
7 });
8}
9
10// For reference: the methods OrderDataSource really has
11// class OrderDataSource {
12// async getOrder(id) { ... }
13// async getOrdersByUserId(userId) { ... }
14// async createOrder(userId, items) { ... }
15// }
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From Debug Lab · Federation 十种典型故障Debug Lab · ten common Federation failures · Federation 考试Federation exam
L2Debug LabDebug Lab故障 2 · Cannot return null for non-nullable fieldFault 2 · Cannot return null for non-nullable field
查一个没有订单的用户，整个 data 变成了null，而且 errors 里有一条很长的消息。
You query a user who has no orders, the whole data turns into null, and errors carries one very long message.
$ node verify-schema.mjs
Query.orders: {"orders":null}
errors: [{
"message": "Cannot return null for non-nullable field Query.orders.",
"path": ["orders"],
"extensions": { "code": "INTERNAL_SERVER_ERROR" }
}]
# 更严重的情况：如果查询是嵌套的，整个 data 会变成 null$ node verify-schema.mjs
Query.orders: {"orders":null}
errors: [{
"message": "Cannot return null for non-nullable field Query.orders.",
"path": ["orders"],
"extensions": { "code": "INTERNAL_SERVER_ERROR" }
}]
# Worse case: if the query is nested, the whole data object turns into null
1async orders(_, { userId }, { dataSources, correlationId }) {
2 const orders = await dataSources.orderDataSource.getOrdersByUserId(userId);
3 return orders; // 数据源可能返回 undefined
4}
5
6// 参考 schema.graphql：
7// type Query {
8// orders(userId: ID!): [Order!]! ← 双重非空
9// }
1async orders(_, { userId }, { dataSources, correlationId }) {
2 const orders = await dataSources.orderDataSource.getOrdersByUserId(userId);
3 return orders; // the data source may return undefined
4}
5
6// For reference, schema.graphql says:
7// type Query {
8// orders(userId: ID!): [Order!]! ← non-null twice
9// }
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this
来自From Debug Lab · Federation 十种典型故障Debug Lab · ten common Federation failures · Federation 考试Federation exam
L2Debug LabDebug Lab故障 4 · PATCH 传了小写状态，返回 500Fault 4 · PATCH sends a lowercase status and gets a 500
Java 那边。mvn test 全过， 但客户端传小写的 shipped 时服务返回 500。
This one is on the Java side. mvn test passes everything, but the service returns 500 when the client sends the lowercase shipped.
$ curl -i -X PATCH localhost:8080/api/orders/1/status \
-H 'Content-Type: application/json' -d '{"status":"shipped"}'
HTTP/1.1 500
{"timestamp":"...","status":500,"error":"Internal Server Error"}
# 服务端日志：
java.lang.IllegalArgumentException: No enum constant
com.techflow.orders.model.OrderStatus.shipped
at java.base/java.lang.Enum.valueOf(Enum.java:293)
at com.techflow.orders.model.OrderStatus.valueOf(OrderStatus.java:3)
at c.t.orders.controller.OrderController.updateOrderStatus(OrderController.java:71)
# mvn test：Tests run: 5, Failures: 0 ← 测试全过$ curl -i -X PATCH localhost:8080/api/orders/1/status \
-H 'Content-Type: application/json' -d '{"status":"shipped"}'
HTTP/1.1 500
{"timestamp":"...","status":500,"error":"Internal Server Error"}
# Server log:
java.lang.IllegalArgumentException: No enum constant
com.techflow.orders.model.OrderStatus.shipped
at java.base/java.lang.Enum.valueOf(Enum.java:293)
at com.techflow.orders.model.OrderStatus.valueOf(OrderStatus.java:3)
at c.t.orders.controller.OrderController.updateOrderStatus(OrderController.java:71)
# mvn test: Tests run: 5, Failures: 0 ← every test passes
1@PatchMapping("/api/orders/{id}/status")
2public ResponseEntity<Order> updateOrderStatus(
3 @PathVariable Long id,
4 @RequestBody Map<String, String> statusUpdate) {
5 OrderStatus status = OrderStatus.valueOf(statusUpdate.get("status"));
6 return ResponseEntity.ok(orderService.updateOrderStatus(id, status));
7}
第 2 步 · 这是什么类型的错误Step 2 · what kind of error is this