2020-04-15 03:15:30 +00:00
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# Union
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2020-05-09 17:53:41 +00:00
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2020-09-01 05:47:22 +00:00
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The definition of a `Union` is similar to an `Interface`, **but with no fields allowed.**.
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The implementation is quite similar for `Async-graphql`; from `Async-graphql`'s perspective, `Union` is a subset of `Interface`.
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2020-05-09 17:53:41 +00:00
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The following example modified the definition of `Interface` a little bit and removed fields.
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```rust
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use async_graphql::*;
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struct Circle {
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radius: f32,
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}
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2020-09-18 00:52:13 +00:00
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#[Object]
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2020-05-09 17:53:41 +00:00
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impl Circle {
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async fn area(&self) -> f32 {
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std::f32::consts::PI * self.radius * self.radius
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}
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async fn scale(&self, s: f32) -> Shape {
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Circle { radius: self.radius * s }.into()
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}
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}
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struct Square {
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width: f32,
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}
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2020-09-18 00:52:13 +00:00
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#[Object]
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2020-05-09 17:53:41 +00:00
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impl Square {
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async fn area(&self) -> f32 {
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self.width * self.width
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}
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async fn scale(&self, s: f32) -> Shape {
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Square { width: self.width * s }.into()
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}
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}
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2020-09-18 00:52:13 +00:00
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#[derive(Union)]
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2020-05-11 04:39:43 +00:00
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enum Shape {
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Circle(Circle),
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Square(Square),
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}
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```
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2020-10-01 02:44:47 +00:00
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## Flattening nested unions
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A restriction in GraphQL is the inability to create a union type out of
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2020-10-01 03:23:24 +00:00
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other union types. All members must be `Object`. To support nested
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2020-10-01 02:44:47 +00:00
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unions, we can "flatten" members that are unions, bringing their members up
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2020-10-01 03:24:30 +00:00
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into the parent union. This is done by applying `#[graphql(flatten)]` on each
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2020-10-01 02:44:47 +00:00
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member we want to flatten.
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```rust
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#[derive(async_graphql::Union)]
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pub enum TopLevelUnion {
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A(A),
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// Will fail to compile unless we flatten the union member
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2020-10-01 03:24:30 +00:00
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#[graphql(flatten)]
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2020-10-01 02:44:47 +00:00
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B(B),
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}
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#[derive(async_graphql::SimpleObject)]
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pub struct A {
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// ...
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}
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#[derive(async_graphql::Union)]
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pub enum B {
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C(C),
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D(D),
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}
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#[derive(async_graphql::SimpleObject)]
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pub struct C {
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// ...
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}
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#[derive(async_graphql::SimpleObject)]
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pub struct D {
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// ...
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}
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```
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The above example transforms the top-level union into this equivalent:
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```rust
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#[derive(async_graphql::Union)]
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pub enum TopLevelUnion {
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A(A),
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C(C),
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D(D),
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}
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```
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