1.9 KiB
1.9 KiB
Union
The definition of a Union
is similar to an Interface
, but with no fields allowed..
The implementation is quite similar for Async-graphql
; from Async-graphql
's perspective, Union
is a subset of Interface
.
The following example modified the definition of Interface
a little bit and removed fields.
use async_graphql::*;
struct Circle {
radius: f32,
}
#[Object]
impl Circle {
async fn area(&self) -> f32 {
std::f32::consts::PI * self.radius * self.radius
}
async fn scale(&self, s: f32) -> Shape {
Circle { radius: self.radius * s }.into()
}
}
struct Square {
width: f32,
}
#[Object]
impl Square {
async fn area(&self) -> f32 {
self.width * self.width
}
async fn scale(&self, s: f32) -> Shape {
Square { width: self.width * s }.into()
}
}
#[derive(Union)]
enum Shape {
Circle(Circle),
Square(Square),
}
Flattening nested unions
A restriction in GraphQL is the inability to create a union type out of
other union types. All members must be Object
. To support nested
unions, we can "flatten" members that are unions, bringing their members up
into the parent union. This is done by applying #[graphql(flatten)]
on each
member we want to flatten.
#[derive(async_graphql::Union)]
pub enum TopLevelUnion {
A(A),
// Will fail to compile unless we flatten the union member
#[graphql(flatten)]
B(B),
}
#[derive(async_graphql::SimpleObject)]
pub struct A {
// ...
}
#[derive(async_graphql::Union)]
pub enum B {
C(C),
D(D),
}
#[derive(async_graphql::SimpleObject)]
pub struct C {
// ...
}
#[derive(async_graphql::SimpleObject)]
pub struct D {
// ...
}
The above example transforms the top-level union into this equivalent:
#[derive(async_graphql::Union)]
pub enum TopLevelUnion {
A(A),
C(C),
D(D),
}