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< h1 id = "unions" > < a class = "header" href = "#unions" > Unions< / a > < / h1 >
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< p > From the server's point of view, < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL unions< / a > are somewhat similar to < a href = "https://spec.graphql.org/June2018/#sec-Interfaces" > interfaces< / a > - the main difference is that they don't contain fields on their own.< / p >
< p > The most obvious and straightforward way to represent a < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL union< / a > in Rust is enum. However, we also can do so either with trait or a regular struct. That's why, for implementing < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL unions< / a > Juniper provides:< / p >
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< ul >
< li > < code > #[derive(GraphQLUnion)]< / code > macro for enums and structs.< / li >
< li > < code > #[graphql_union]< / code > for traits.< / li >
< / ul >
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< h2 id = "enums" > < a class = "header" href = "#enums" > Enums< / a > < / h2 >
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< p > Most of the time, we just need a trivial and straightforward Rust enum to represent a < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL union< / a > .< / p >
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< pre > < pre class = "playground" > < code class = "language-rust" > < span class = "boring" > extern crate juniper;
< / span > < span class = "boring" > extern crate derive_more;
< / span > use derive_more::From;
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use juniper::{GraphQLObject, GraphQLUnion};
#[derive(GraphQLObject)]
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struct Human {
id: String,
home_planet: String,
}
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#[derive(GraphQLObject)]
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struct Droid {
id: String,
primary_function: String,
}
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#[derive(From, GraphQLUnion)]
enum Character {
Human(Human),
Droid(Droid),
}
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< span class = "boring" >
< / span > < span class = "boring" > fn main() {}
< / span > < / code > < / pre > < / pre >
< h3 id = "ignoring-enum-variants" > < a class = "header" href = "#ignoring-enum-variants" > Ignoring enum variants< / a > < / h3 >
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< p > In some rare situations we may want to omit exposing an enum variant in the GraphQL schema.< / p >
< p > As an example, let's consider the situation where we need to bind some type parameter < code > T< / code > for doing interesting type-level stuff in our resolvers. To achieve this we need to have < code > PhantomData< T> < / code > , but we don't want it exposed in the GraphQL schema.< / p >
< blockquote >
< p > < strong > WARNING< / strong > :< br / >
It's the < em > library user's responsibility< / em > to ensure that ignored enum variant is < em > never< / em > returned from resolvers, otherwise resolving the GraphQL query will < strong > panic at runtime< / strong > .< / p >
< / blockquote >
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< pre > < pre class = "playground" > < code class = "language-rust" > < span class = "boring" > extern crate juniper;
< / span > < span class = "boring" > extern crate derive_more;
< / span > < span class = "boring" > use std::marker::PhantomData;
< / span > use derive_more::From;
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use juniper::{GraphQLObject, GraphQLUnion};
#[derive(GraphQLObject)]
struct Human {
id: String,
home_planet: String,
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}
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#[derive(GraphQLObject)]
struct Droid {
id: String,
primary_function: String,
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}
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#[derive(From, GraphQLUnion)]
enum Character< S> {
Human(Human),
Droid(Droid),
#[from(ignore)]
#[graphql(ignore)] // or `#[graphql(skip)]`, your choice
_State(PhantomData< S> ),
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}
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< span class = "boring" >
< / span > < span class = "boring" > fn main() {}
< / span > < / code > < / pre > < / pre >
< h3 id = "external-resolver-functions" > < a class = "header" href = "#external-resolver-functions" > External resolver functions< / a > < / h3 >
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< p > If some custom logic is needed to resolve a < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL union< / a > variant, you may specify an external function to do so:< / p >
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< pre > < pre class = "playground" > < code class = "language-rust" > < span class = "boring" > #![allow(dead_code)]
< / span > < span class = "boring" > extern crate juniper;
< / span > use juniper::{GraphQLObject, GraphQLUnion};
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#[derive(GraphQLObject)]
#[graphql(Context = CustomContext)]
struct Human {
id: String,
home_planet: String,
}
#[derive(GraphQLObject)]
#[graphql(Context = CustomContext)]
struct Droid {
id: String,
primary_function: String,
}
pub struct CustomContext {
droid: Droid,
}
impl juniper::Context for CustomContext {}
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#[derive(GraphQLUnion)]
#[graphql(Context = CustomContext)]
enum Character {
Human(Human),
#[graphql(with = Character::droid_from_context)]
Droid(Droid),
}
impl Character {
// NOTICE: The function signature must contain `& self` and `& Context`,
// and return `Option< & VariantType> `.
fn droid_from_context< 'c> (& self, ctx: & 'c CustomContext) -> Option< & 'c Droid> {
Some(& ctx.droid)
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}
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}
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< span class = "boring" >
< / span > < span class = "boring" > fn main() {}
< / span > < / code > < / pre > < / pre >
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< p > With an external resolver function we can even declare a new < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL union< / a > variant where the Rust type is absent in the initial enum definition. The attribute syntax < code > #[graphql(on VariantType = resolver_fn)]< / code > follows the < a href = "https://spec.graphql.org/June2018/#example-f8163" > GraphQL syntax for dispatching union variants< / a > .< / p >
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< pre > < pre class = "playground" > < code class = "language-rust" > < span class = "boring" > #![allow(dead_code)]
< / span > < span class = "boring" > extern crate juniper;
< / span > use juniper::{GraphQLObject, GraphQLUnion};
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#[derive(GraphQLObject)]
#[graphql(Context = CustomContext)]
struct Human {
id: String,
home_planet: String,
}
#[derive(GraphQLObject)]
#[graphql(Context = CustomContext)]
struct Droid {
id: String,
primary_function: String,
}
#[derive(GraphQLObject)]
#[graphql(Context = CustomContext)]
struct Ewok {
id: String,
is_funny: bool,
}
pub struct CustomContext {
ewok: Ewok,
}
impl juniper::Context for CustomContext {}
#[derive(GraphQLUnion)]
#[graphql(Context = CustomContext)]
#[graphql(on Ewok = Character::ewok_from_context)]
enum Character {
Human(Human),
Droid(Droid),
#[graphql(ignore)] // or `#[graphql(skip)]`, your choice
Ewok,
}
impl Character {
fn ewok_from_context< 'c> (& self, ctx: & 'c CustomContext) -> Option< & 'c Ewok> {
if let Self::Ewok = self {
Some(& ctx.ewok)
} else {
None
}
}
}
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< span class = "boring" >
< / span > < span class = "boring" > fn main() {}
< / span > < / code > < / pre > < / pre >
< h2 id = "structs" > < a class = "header" href = "#structs" > Structs< / a > < / h2 >
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< p > Using Rust structs as < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL unions< / a > is very similar to using enums, with the nuance that specifying an external resolver function is the only way to declare a < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL union< / a > variant.< / p >
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< pre > < pre class = "playground" > < code class = "language-rust" > < span class = "boring" > extern crate juniper;
< / span > < span class = "boring" > use std::collections::HashMap;
< / span > use juniper::{GraphQLObject, GraphQLUnion};
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#[derive(GraphQLObject)]
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#[graphql(Context = Database)]
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struct Human {
id: String,
home_planet: String,
}
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#[derive(GraphQLObject)]
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#[graphql(Context = Database)]
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struct Droid {
id: String,
primary_function: String,
}
struct Database {
humans: HashMap< String, Human> ,
droids: HashMap< String, Droid> ,
}
impl juniper::Context for Database {}
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#[derive(GraphQLUnion)]
#[graphql(
Context = Database,
on Human = Character::get_human,
on Droid = Character::get_droid,
)]
struct Character {
id: String,
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}
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impl Character {
fn get_human< 'db> (& self, ctx: & 'db Database) -> Option< & 'db Human> {
ctx.humans.get(& self.id)
}
fn get_droid< 'db> (& self, ctx: & 'db Database) -> Option< & 'db Droid> {
ctx.droids.get(& self.id)
}
}
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< span class = "boring" >
< / span > < span class = "boring" > fn main() {}
< / span > < / code > < / pre > < / pre >
< h2 id = "traits" > < a class = "header" href = "#traits" > Traits< / a > < / h2 >
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< p > To use a Rust trait definition as a < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL union< / a > you need to use the < code > #[graphql_union]< / code > macro. < a href = "https://doc.rust-lang.org/stable/reference/procedural-macros.html#derive-macros" > Rust doesn't allow derive macros on traits< / a > , so using < code > #[derive(GraphQLUnion)]< / code > on traits doesn't work.< / p >
< blockquote >
< p > < strong > NOTICE< / strong > :< br / >
A < strong > trait has to be < a href = "https://doc.rust-lang.org/stable/reference/items/traits.html#object-safety" > object safe< / a > < / strong > , because schema resolvers will need to return a < a href = "https://doc.rust-lang.org/stable/reference/types/trait-object.html" > trait object< / a > to specify a < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL union< / a > behind it.< / p >
< / blockquote >
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< pre > < pre class = "playground" > < code class = "language-rust" > < span class = "boring" > extern crate juniper;
< / span > use juniper::{graphql_union, GraphQLObject};
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#[derive(GraphQLObject)]
struct Human {
id: String,
home_planet: String,
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}
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#[derive(GraphQLObject)]
struct Droid {
id: String,
primary_function: String,
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}
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#[graphql_union]
trait Character {
// NOTICE: The method signature must contain `& self` and return `Option< & VariantType> `.
fn as_human(& self) -> Option< & Human> { None }
fn as_droid(& self) -> Option< & Droid> { None }
}
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impl Character for Human {
fn as_human(& self) -> Option< & Human> { Some(& self) }
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}
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impl Character for Droid {
fn as_droid(& self) -> Option< & Droid> { Some(& self) }
}
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< span class = "boring" >
< / span > < span class = "boring" > fn main() {}
< / span > < / code > < / pre > < / pre >
< h3 id = "custom-context" > < a class = "header" href = "#custom-context" > Custom context< / a > < / h3 >
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< p > If a < a href = "https://docs.rs/juniper/0.14.2/juniper/trait.Context.html" > < code > Context< / code > < / a > is required in a trait method to resolve a < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL union< / a > variant, specify it as an argument.< / p >
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< pre > < pre class = "playground" > < code class = "language-rust" > < span class = "boring" > #![allow(unused_variables)]
< / span > < span class = "boring" > extern crate juniper;
< / span > < span class = "boring" > use std::collections::HashMap;
< / span > use juniper::{graphql_union, GraphQLObject};
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#[derive(GraphQLObject)]
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#[graphql(Context = Database)]
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struct Human {
id: String,
home_planet: String,
}
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#[derive(GraphQLObject)]
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#[graphql(Context = Database)]
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struct Droid {
id: String,
primary_function: String,
}
struct Database {
humans: HashMap< String, Human> ,
droids: HashMap< String, Droid> ,
}
impl juniper::Context for Database {}
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#[graphql_union(context = Database)]
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trait Character {
// NOTICE: The method signature may optionally contain `& Context`.
fn as_human< 'db> (& self, ctx: & 'db Database) -> Option< & 'db Human> { None }
fn as_droid< 'db> (& self, ctx: & 'db Database) -> Option< & 'db Droid> { None }
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}
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impl Character for Human {
fn as_human< 'db> (& self, ctx: & 'db Database) -> Option< & 'db Human> {
ctx.humans.get(& self.id)
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}
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}
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impl Character for Droid {
fn as_droid< 'db> (& self, ctx: & 'db Database) -> Option< & 'db Droid> {
ctx.droids.get(& self.id)
}
}
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< span class = "boring" >
< / span > < span class = "boring" > fn main() {}
< / span > < / code > < / pre > < / pre >
< h3 id = "ignoring-trait-methods" > < a class = "header" href = "#ignoring-trait-methods" > Ignoring trait methods< / a > < / h3 >
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< p > As with enums, we may want to omit some trait methods to be assumed as < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL union< / a > variants and ignore them.< / p >
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< pre > < pre class = "playground" > < code class = "language-rust" > < span class = "boring" > extern crate juniper;
< / span > use juniper::{graphql_union, GraphQLObject};
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#[derive(GraphQLObject)]
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struct Human {
id: String,
home_planet: String,
}
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#[derive(GraphQLObject)]
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struct Droid {
id: String,
primary_function: String,
}
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#[graphql_union]
trait Character {
fn as_human(& self) -> Option< & Human> { None }
fn as_droid(& self) -> Option< & Droid> { None }
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#[graphql(ignore)] // or `#[graphql(skip)]`, your choice
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fn id(& self) -> & str;
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}
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impl Character for Human {
fn as_human(& self) -> Option< & Human> { Some(& self) }
fn id(& self) -> & str { self.id.as_str() }
}
impl Character for Droid {
fn as_droid(& self) -> Option< & Droid> { Some(& self) }
fn id(& self) -> & str { self.id.as_str() }
}
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< span class = "boring" >
< / span > < span class = "boring" > fn main() {}
< / span > < / code > < / pre > < / pre >
< h3 id = "external-resolver-functions-1" > < a class = "header" href = "#external-resolver-functions-1" > External resolver functions< / a > < / h3 >
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< p > Similarly to enums and structs, it's not mandatory to use trait methods as < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL union< / a > variant resolvers. Instead, custom functions may be specified:< / p >
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< pre > < pre class = "playground" > < code class = "language-rust" > < span class = "boring" > extern crate juniper;
< / span > < span class = "boring" > use std::collections::HashMap;
< / span > use juniper::{graphql_union, GraphQLObject};
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#[derive(GraphQLObject)]
#[graphql(Context = Database)]
struct Human {
id: String,
home_planet: String,
}
#[derive(GraphQLObject)]
#[graphql(Context = Database)]
struct Droid {
id: String,
primary_function: String,
}
struct Database {
humans: HashMap< String, Human> ,
droids: HashMap< String, Droid> ,
}
impl juniper::Context for Database {}
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#[graphql_union(context = Database)]
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#[graphql_union(
on Human = DynCharacter::get_human,
on Droid = get_droid,
)]
trait Character {
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#[graphql(ignore)] // or `#[graphql(skip)]`, your choice
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fn id(& self) -> & str;
}
impl Character for Human {
fn id(& self) -> & str { self.id.as_str() }
}
impl Character for Droid {
fn id(& self) -> & str { self.id.as_str() }
}
// The trait object is always `Send` and `Sync`.
type DynCharacter< 'a> = dyn Character + Send + Sync + 'a;
impl< 'a> DynCharacter< 'a> {
fn get_human< 'db> (& self, ctx: & 'db Database) -> Option< & 'db Human> {
ctx.humans.get(self.id())
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}
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}
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// External resolver function doesn't have to be a method of a type.
// It's only a matter of the function signature to match the requirements.
fn get_droid< 'db> (ch: & DynCharacter< '_> , ctx: & 'db Database) -> Option< & 'db Droid> {
ctx.droids.get(ch.id())
}
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< span class = "boring" >
< / span > < span class = "boring" > fn main() {}
< / span > < / code > < / pre > < / pre >
< h2 id = "scalarvalue-considerations" > < a class = "header" href = "#scalarvalue-considerations" > < code > ScalarValue< / code > considerations< / a > < / h2 >
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< p > By default, < code > #[derive(GraphQLUnion)]< / code > and < code > #[graphql_union]< / code > macros generate code, which is generic over a < a href = "https://docs.rs/juniper/latest/juniper/trait.ScalarValue.html" > < code > ScalarValue< / code > < / a > type. This may introduce a problem when at least one of < a href = "https://spec.graphql.org/June2018/#sec-Unions" > GraphQL union< / a > variants is restricted to a concrete < a href = "https://docs.rs/juniper/latest/juniper/trait.ScalarValue.html" > < code > ScalarValue< / code > < / a > type in its implementation. To resolve such problem, a concrete < a href = "https://docs.rs/juniper/latest/juniper/trait.ScalarValue.html" > < code > ScalarValue< / code > < / a > type should be specified:< / p >
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< pre > < pre class = "playground" > < code class = "language-rust" > < span class = "boring" > #![allow(dead_code)]
< / span > < span class = "boring" > extern crate juniper;
< / span > use juniper::{DefaultScalarValue, GraphQLObject, GraphQLUnion};
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#[derive(GraphQLObject)]
#[graphql(Scalar = DefaultScalarValue)]
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struct Human {
id: String,
home_planet: String,
}
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#[derive(GraphQLObject)]
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struct Droid {
id: String,
primary_function: String,
}
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#[derive(GraphQLUnion)]
#[graphql(Scalar = DefaultScalarValue)] // removing this line will fail compilation
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enum Character {
Human(Human),
Droid(Droid),
}
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< span class = "boring" >
< / span > < span class = "boring" > fn main() {}
< / span > < / code > < / pre > < / pre >
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< / main >
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