async-graphql/docs/en/src/define_union.md

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()
    }
}

#[Union]
enum Shape {
    Circle(Circle),
    Square(Square),
}