2020-05-13 04:49:43 +00:00
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# 错误扩展
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引用 [graphql-spec](https://spec.graphql.org/June2018/#example-fce18)
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> GraphQL服务可以通过扩展提供错误的附加条目。
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> 该条目(如果设置)必须是一个映射作为其值,用于附加错误的其它信息。
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## 示例
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我建议您查看此 [错误扩展示例](https://github.com/async-graphql/examples/blob/master/actix-web/error-extensions/src/main.rs) 作为快速入门。
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## 一般概念
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在`Async-graphql`中,所有面向用户的错误都强制转换为`FieldError`类型,默认情况下会提供
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由`std:::fmt::Display`暴露的错误消息。但是,`FieldError`实际上提供了一个额外的
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字段`Option<serde_json::Value>`,如果它是一个`serde_json::Map`,则可以扩展错误的信息。
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Resolver函数类似这样:
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```rust
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async fn parse_with_extensions(&self) -> Result<i32, FieldError> {
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let my_extension = json!({ "details": "CAN_NOT_FETCH" });
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Err(FieldError("MyMessage", Some(my_extension)))
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}
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```
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然后可以返回如下响应:
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```json
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{
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"errors": [
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{
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"message": "MyMessage",
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"locations": [ ... ],
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"path": [ ... ],
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"extensions": {
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"details": "CAN_NOT_FETCH",
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}
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}
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]
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}
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```
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## ErrorExtensions
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手动构造新的`FieldError`很麻烦。这就是为什么`Async-graphql`提供
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两个方便特性,可将您的错误转换为适当的`FieldError`扩展。
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扩展任何错误的最简单方法是对错误调用`extend_with`。
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这将把任何错误转换为具有给定扩展信息的`FieldError`。
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```rust
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use std::num::ParseIntError;
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async fn parse_with_extensions(&self) -> FieldResult<i32> {
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Ok("234a"
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.parse()
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.map_err(|err: ParseIntError| err.extend_with(|_err| json!({"code": 404})))?)
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}
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```
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### 为自定义错误实现ErrorExtensions
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你也可以给自己的错误类型实现`ErrorExtensions`:
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```rust
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#[macro_use]
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extern crate thiserror;
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#[derive(Debug, Error)]
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pub enum MyError {
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#[error("Could not find resource")]
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NotFound,
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#[error("ServerError")]
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ServerError(String),
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#[error("No Extensions")]
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ErrorWithoutExtensions,
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}
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impl ErrorExtensions for MyError {
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// lets define our base extensions
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fn extend(&self) -> FieldError {
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let extensions = match self {
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MyError::NotFound => json!({"code": "NOT_FOUND"}),
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MyError::ServerError(reason) => json!({ "reason": reason }),
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MyError::ErrorWithoutExtensions => {
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json!("This will be ignored since it does not represent an object.")
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}
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};
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FieldError(format!("{}", self), Some(extensions))
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}
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}
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```
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您只需要对错误调用`extend`即可将错误与其提供的扩展信息一起传递,或者通过`extend_with`进一步扩展错误信息。
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```rust
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async fn parse_with_extensions_result(&self) -> FieldResult<i32> {
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// Err(MyError::NotFound.extend())
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// OR
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Err(MyError::NotFound.extend_with(|_| json!({ "on_the_fly": "some_more_info" })))
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}
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```
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```json
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{
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"errors": [
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{
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"message": "NotFound",
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"locations": [ ... ],
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"path": [ ... ],
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"extensions": {
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"code": "NOT_FOUND",
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"on_the_fly": "some_more_info"
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}
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}
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]
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}
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```
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## ResultExt
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这个特质使您可以直接在结果上调用`extend_err`。因此上面的代码不再那么冗长。
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```rust
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use async_graphql::*;
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async fn parse_with_extensions(&self) -> FieldResult<i32> {
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Ok("234a"
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.parse()
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.extend_err(|_| json!({"code": 404}))?)
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}
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```
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### 链式调用
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由于对所有`&E where E: std::fmt::Display`实现了`ErrorExtensions`和`ResultsExt`,我们可以将扩展链接在一起。
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```rust
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use async_graphql::*;
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async fn parse_with_extensions(&self) -> FieldResult<i32> {
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match "234a".parse() {
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Ok(n) => Ok(n),
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Err(e) => Err(e
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.extend_with(|_| json!({"code": 404}))
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.extend_with(|_| json!({"details": "some more info.."}))
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// keys may also overwrite previous keys...
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.extend_with(|_| json!({"code": 500}))),
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}
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}
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```
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响应:
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```json
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{
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"errors": [
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{
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"message": "MyMessage",
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"locations": [ ... ],
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"path": [ ... ],
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"extensions": {
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"details": "some more info...",
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"code": 500,
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}
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}
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]
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}
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```
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### 陷阱
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Rust的稳定版本还未提供特化功能,这就是为什么`ErrorExtensions`为`&E where E: std::fmt::Display`实现,代替`E:std::fmt::Display`通过提供一些特化功能。
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[Autoref-based stable specialization](https://github.com/dtolnay/case-studies/blob/master/autoref-specialization/README.md).
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缺点是下面的代码**不能**编译:
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```rust,ignore,does_not_compile
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async fn parse_with_extensions_result(&self) -> FieldResult<i32> {
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// the trait `error::ErrorExtensions` is not implemented
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// for `std::num::ParseIntError`
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"234a".parse().extend_err(|_| json!({"code": 404}))
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}
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```
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但这可以通过编译:
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```rust,ignore,does_not_compile
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async fn parse_with_extensions_result(&self) -> FieldResult<i32> {
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// does work because ErrorExtensions is implemented for &ParseIntError
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"234a"
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.parse()
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.map_err(|ref e: ParseIntError| e.extend_with(|_| json!({"code": 404})))
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}
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```
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