派生字段

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。

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。


#![allow(unused)]
fn main() {
extern crate chrono;
use chrono::Utc;
extern crate async_graphql;
use async_graphql::*;
struct DateRFC3339(chrono::DateTime<Utc>);
struct DateRFC2822(chrono::DateTime<Utc>);

#[Scalar]
impl ScalarType for DateRFC3339 {
  fn parse(value: Value) -> InputValueResult<Self> { todo!() } 

  fn to_value(&self) -> Value {
    Value::String(self.0.to_rfc3339())
  }
}

#[Scalar]
impl ScalarType for DateRFC2822 {
  fn parse(value: Value) -> InputValueResult<Self> { todo!() } 

  fn to_value(&self) -> Value {
    Value::String(self.0.to_rfc2822())
  }
}

impl From<DateRFC2822> for DateRFC3339 {
    fn from(value: DateRFC2822) -> Self {
      DateRFC3339(value.0)
    }
}

struct Query;

#[Object]
impl Query {
    #[graphql(derived(name = "date_rfc3339", into = "DateRFC3339"))]
    async fn duration_rfc2822(&self, arg: String) -> DateRFC2822 {
        todo!()
    }
}
}

它将呈现为如下GraphQL:

type Query {
	duration_rfc2822(arg: String): DateRFC2822!
	duration_rfc3339(arg: String): DateRFC3339!
}

包装类型

因为 孤儿规则,以下代码无法通过编译:

impl From<Vec<U>> for Vec<T> {
  ...
}

因此,你将无法为现有的包装类型结构(如VecOption)生成派生字段。 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>>. 使用 with 参数来定义一个转换函数,而不是用 Into::into

Example


#![allow(unused)]
fn main() {
extern crate serde;
use serde::{Serialize, Deserialize};
extern crate async_graphql;
use async_graphql::*;
#[derive(Serialize, Deserialize, Clone)]
struct ValueDerived(String);

#[derive(Serialize, Deserialize, Clone)]
struct ValueDerived2(String);

scalar!(ValueDerived);
scalar!(ValueDerived2);

impl From<ValueDerived> for ValueDerived2 {
    fn from(value: ValueDerived) -> Self {
        ValueDerived2(value.0)
    }
}

fn option_to_option<T, U: From<T>>(value: Option<T>) -> Option<U> {
    value.map(|x| x.into())
}

#[derive(SimpleObject)]
struct TestObj {
    #[graphql(derived(owned, name = "value2", into = "Option<ValueDerived2>", with = "option_to_option"))]
    pub value1: Option<ValueDerived>,
}
}