97 lines
2.3 KiB
Markdown
97 lines
2.3 KiB
Markdown
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# 派生字段
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有时两个字段有一样的查询逻辑，仅仅是输出的类型不同，在 `async-graphql` 中，你可以为它创建派生字段。
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在以下例子中，你已经有一个`duration_rfc2822`字段输出`RFC2822`格式的时间格式，然后复用它派生一个新的`date_rfc3339`字段。
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```rust
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struct DateRFC3339(chrono::DateTime);
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struct DateRFC2822(chrono::DateTime);
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#[Scalar]
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impl ScalarType for DateRFC3339 {
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fn parse(value: Value) -> InputValueResult { ... }
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fn to_value(&self) -> Value {
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Value::String(self.0.to_rfc3339())
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}
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}
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#[Scalar]
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impl ScalarType for DateRFC2822 {
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fn parse(value: Value) -> InputValueResult { ... }
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fn to_value(&self) -> Value {
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Value::String(self.0.to_rfc2822())
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}
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}
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impl From<DateRFC2822> for DateRFC3339 {
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fn from(value: DateRFC2822) -> Self {
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DateRFC3339(value.0)
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}
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}
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struct Query;
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#[Object]
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impl Query {
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#[graphql(derived(name = "date_rfc3339", into = "DateRFC3339"))]
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async fn duration_rfc2822(&self, arg: String) -> DateRFC2822 {
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todo!()
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}
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}
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```
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它将呈现为如下GraphQL：
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```graphql
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type Query {
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duration_rfc2822(arg: String): DateRFC2822!
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duration_rfc3339(arg: String): DateRFC3339!
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}
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```
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## 包装类型
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因为 [孤儿规则](https://doc.rust-lang.org/book/traits.html#rules-for-implementing-traits)，以下代码无法通过编译：
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```
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impl From<Vec<U>> for Vec<T> {
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...
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}
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```
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因此，你将无法为现有的包装类型结构（如`Vec`或`Option`）生成派生字段。
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但是当你为 `T` 实现了 `From<U>` 后，你可以为 `Vec<T>` 实现 `From<Vec<U>>`，为 `Option<T>` 实现 `From<Option<U>>`.
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使用 `with` 参数来定义一个转换函数，而不是用 `Into::into`。
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### Example
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```rust
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#[derive(Serialize, Deserialize, Clone)]
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struct ValueDerived(String);
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#[derive(Serialize, Deserialize, Clone)]
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struct ValueDerived2(String);
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scalar!(ValueDerived);
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scalar!(ValueDerived2);
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impl From<ValueDerived> for ValueDerived2 {
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fn from(value: ValueDerived) -> Self {
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ValueDerived2(value.0)
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}
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}
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fn option_to_option<T, U: From<T>>(value: Option<T>) -> Option<U> {
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value.map(|x| x.into())
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}
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#[derive(SimpleObject)]
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struct TestObj {
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#[graphql(derived(owned, name = "value2", into = "Option<ValueDerived2>", with = "option_to_option"))]
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pub value1: Option<ValueDerived>,
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}
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```
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