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< ol class = "chapter" > < li class = "chapter-item expanded " > < a href = "../introduction.html" > < strong aria-hidden = "true" > 1.< / strong > Introduction< / a > < / li > < li class = "chapter-item expanded " > < a href = "../quickstart.html" > < strong aria-hidden = "true" > 2.< / strong > Quickstart< / a > < / li > < li class = "chapter-item expanded " > < a href = "../types/index.html" > < strong aria-hidden = "true" > 3.< / strong > Type system< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../types/objects/index.html" > < strong aria-hidden = "true" > 3.1.< / strong > Objects< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../types/objects/complex_fields.html" > < strong aria-hidden = "true" > 3.1.1.< / strong > Complex fields< / a > < / li > < li class = "chapter-item expanded " > < a href = "../types/objects/context.html" > < strong aria-hidden = "true" > 3.1.2.< / strong > Context< / a > < / li > < li class = "chapter-item expanded " > < a href = "../types/objects/error/index.html" > < strong aria-hidden = "true" > 3.1.3.< / strong > Error handling< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../types/objects/error/field.html" > < strong aria-hidden = "true" > 3.1.3.1.< / strong > Field errors< / a > < / li > < li class = "chapter-item expanded " > < a href = "../types/objects/error/schema.html" > < strong aria-hidden = "true" > 3.1.3.2.< / strong > Schema errors< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../types/objects/generics.html" > < strong aria-hidden = "true" > 3.1.4.< / strong > Generics< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../types/interfaces.html" > < strong aria-hidden = "true" > 3.2.< / strong > Interfaces< / a > < / li > < li class = "chapter-item expanded " > < a href = "../types/unions.html" class = "active" > < strong aria-hidden = "true" > 3.3.< / strong > Unions< / a > < / li > < li class = "chapter-item expanded " > < a href = "../types/enums.html" > < strong aria-hidden = "true" > 3.4.< / strong > Enums< / a > < / li > < li class = "chapter-item expanded " > < a href = "../types/input_objects.html" > < strong aria-hidden = "true" > 3.5.< / strong > Input objects< / a > < / li > < li class = "chapter-item expanded " > < a href = "../types/scalars.html" > < strong aria-hidden = "true" > 3.6.< / strong > Scalars< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../schema/index.html" > < strong aria-hidden = "true" > 4.< / strong > Schema< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../schema/subscriptions.html" > < strong aria-hidden = "true" > 4.1.< / strong > Subscriptions< / a > < / li > < li class = "chapter-item expanded " > < a href = "../schema/introspection.html" > < strong aria-hidden = "true" > 4.2.< / strong > Introspection< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../serve/index.html" > < strong aria-hidden = "true" > 5.< / strong > Serving< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../serve/batching.html" > < strong aria-hidden = "true" > 5.1.< / strong > Batching< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../advanced/index.html" > < strong aria-hidden = "true" > 6.< / strong > Advanced Topics< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../advanced/implicit_and_explicit_null.html" > < strong aria-hidden = "true" > 6.1.< / strong > Implicit and explicit null< / a > < / li > < li class = "chapter-item expanded " > < a href = "../advanced/n_plus_1.html" > < strong aria-hidden = "true" > 6.2.< / strong > N+1 problem< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../advanced/dataloader.html" > < strong aria-hidden = "true" > 6.2.1.< / strong > DataLoader< / a > < / li > < li class = "chapter-item expanded " > < a href = "../advanced/lookahead.html" > < strong aria-hidden = "true" > 6.2.2.< / strong > Look-ahead< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../advanced/eager_loading.html" > < strong aria-hidden = "true" > 6.2.2.1.< / strong > Eager loading< / a > < / li > < / ol > < / li > < / ol > < / li > < / ol > < / li > < / ol >
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< h1 id = "unions" > < a class = "header" href = "#unions" > Unions< / a > < / h1 >
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< blockquote >
< p > < a href = "https://spec.graphql.org/October2021#sec-Unions" > GraphQL unions< / a > represent an object that could be one of a list of < a href = "https://spec.graphql.org/October2021#sec-Objects" > GraphQL object< / a > types, but provides for no guaranteed fields between those types. They also differ from < a href = "https://spec.graphql.org/October2021#sec-Interfaces" > interfaces< / a > in that < a href = "https://spec.graphql.org/October2021#sec-Objects" > object< / a > types declare what < a href = "https://spec.graphql.org/October2021#sec-Interfaces" > interfaces< / a > they implement, but are not aware of what < a href = "https://spec.graphql.org/October2021#sec-Unions" > unions< / a > contain them.< / p >
< / blockquote >
< p > From the server's point of view, < a href = "https://spec.graphql.org/October2021#sec-Unions" > GraphQL unions< / a > are somewhat similar to < a href = "https://spec.graphql.org/October2021#sec-Interfaces" > interfaces< / a > : the main difference is that they don't contain fields on their own, and so, we only need to represent a value, < em > dispatchable< / em > into concrete < a href = "https://spec.graphql.org/October2021#sec-Objects" > objects< / a > .< / p >
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< p > Obviously, the most straightforward approach to express < a href = "https://spec.graphql.org/October2021#sec-Unions" > GraphQL unions< / a > in < a href = "https://www.rust-lang.org" > Rust< / a > is to use < a href = "https://doc.rust-lang.org/reference/items/enumerations.html#enumerations" > enums< / a > . In < a href = "https://docs.rs/juniper" > Juniper< / a > this may be done by using < a href = "https://docs.rs/juniper/0.16.0/juniper/derive.GraphQLUnion.html" > < code > #[derive(GraphQLInterface)]< / code > < / a > attribute on them:< / p >
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< pre > < pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > extern crate derive_more;
< / span > < span class = "boring" > extern crate juniper;
< / span > < span class = "boring" > use derive_more::From;
< / span > < span class = "boring" > use juniper::{GraphQLObject, GraphQLUnion};
< / span > < span class = "boring" >
< / span > #[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)]
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// ^^^^ only for convenience, and may be omitted
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enum Character {
Human(Human),
Droid(Droid),
}
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< span class = "boring" >
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< / span > < span class = "boring" > fn main() {}< / span > < / code > < / pre > < / pre >
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< h3 id = "renaming" > < a class = "header" href = "#renaming" > Renaming< / a > < / h3 >
< p > Just as with < a href = "objects/index.html#renaming" > renaming GraphQL objects< / a > , we can override the default < a href = "https://spec.graphql.org/October2021#sec-Unions" > union< / a > name by using the < code > #[graphql(name = "...")]< / code > attribute:< / p >
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< pre > < pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > extern crate juniper;
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< / span > < span class = "boring" > use juniper::{GraphQLObject, GraphQLUnion};
< / span > < span class = "boring" >
< / span > < span class = "boring" > #[derive(GraphQLObject)]
< / span > < span class = "boring" > struct Human {
< / span > < span class = "boring" > id: String,
< / span > < span class = "boring" > home_planet: String,
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > < span class = "boring" > #[derive(GraphQLObject)]
< / span > < span class = "boring" > struct Droid {
< / span > < span class = "boring" > id: String,
< / span > < span class = "boring" > primary_function: String,
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > #[derive(GraphQLUnion)]
#[graphql(name = "CharacterUnion")]
enum Character { // exposed as `CharacterUnion` in GraphQL schema
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Human(Human),
Droid(Droid),
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}
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< span class = "boring" >
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< / span > < span class = "boring" > fn main() {}< / span > < / code > < / pre > < / pre >
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< blockquote >
< p > < strong > NOTE< / strong > : Unlike < a href = "https://doc.rust-lang.org/reference/items/enumerations.html#enumerations" > Rust enum variants< / a > , < a href = "https://spec.graphql.org/October2021#sec-Unions" > GraphQL union members< / a > don't have any special names aside from the ones provided by < a href = "https://spec.graphql.org/October2021#sec-Objects" > objects< / a > themselves, and so, obviously, < strong > cannot be renamed< / strong > .< / p >
< / blockquote >
< h3 id = "documentation" > < a class = "header" href = "#documentation" > Documentation< / a > < / h3 >
< p > Similarly to < a href = "objects/index.html#documentation" > documenting GraphQL objects< / a > , we can < a href = "https://spec.graphql.org/October2021#sec-Descriptions" > document< / a > a < a href = "https://spec.graphql.org/October2021#sec-Unions" > GraphQL union< / a > via < code > #[graphql(description = "...")]< / code > attribute or < a href = "https://doc.rust-lang.org/reference/comments.html#doc-comments" > Rust doc comments< / a > :< / p >
< pre > < pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > extern crate juniper;
< / span > < span class = "boring" > use juniper::{GraphQLObject, GraphQLUnion};
< / span > < span class = "boring" >
< / span > < span class = "boring" > #[derive(GraphQLObject)]
< / span > < span class = "boring" > struct Human {
< / span > < span class = "boring" > id: String,
< / span > < span class = "boring" > home_planet: String,
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > < span class = "boring" > #[derive(GraphQLObject)]
< / span > < span class = "boring" > struct Droid {
< / span > < span class = "boring" > id: String,
< / span > < span class = "boring" > primary_function: String,
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > /// This doc comment is visible in both Rust API docs and GraphQL schema
/// descriptions.
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#[derive(GraphQLUnion)]
enum Character {
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/// This doc comment is visible only in Rust API docs.
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Human(Human),
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/// This doc comment is visible only in Rust API docs.
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Droid(Droid),
}
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/// This doc comment is visible only in Rust API docs.
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#[derive(GraphQLUnion)]
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#[graphql(description = "This description overwrites the one from doc comment.")]
// ^^^^^^^^^^^ or `desc` shortcut, up to your preference
enum Person {
/// This doc comment is visible only in Rust API docs.
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Human(Human),
}
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< span class = "boring" >
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< / span > < span class = "boring" > fn main() {}< / span > < / code > < / pre > < / pre >
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< blockquote >
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< p > < strong > NOTE< / strong > : Unlike < a href = "https://doc.rust-lang.org/reference/items/enumerations.html#enumerations" > Rust enum variants< / a > , < a href = "https://spec.graphql.org/October2021#sec-Unions" > GraphQL union members< / a > don't have any special constructors aside from the provided < a href = "https://spec.graphql.org/October2021#sec-Objects" > objects< / a > directly, and so, < strong > cannot be < a href = "https://spec.graphql.org/October2021#sec-Descriptions" > documented< / a > < / strong > , but rather reuse < a href = "https://spec.graphql.org/October2021#sec-Descriptions" > object descriptions< / a > "as is".< / p >
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< / blockquote >
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< h3 id = "ignoring" > < a class = "header" href = "#ignoring" > Ignoring< / a > < / h3 >
< p > In some rare situations we may want to omit exposing an < a href = "https://doc.rust-lang.org/reference/items/enumerations.html#enumerations" > enum< / a > variant in a < a href = "https://graphql.org/learn/schema" > GraphQL schema< / a > . < a href = "enums.html#ignoring" > Similarly to GraphQL enums< / a > , we can just annotate the variant with the < code > #[graphql(ignore)]< / code > attribute.< / 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 >
< pre > < pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > extern crate derive_more;
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< / span > < span class = "boring" > extern crate juniper;
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< / span > < span class = "boring" > use std::marker::PhantomData;
< / span > < span class = "boring" > use derive_more::From;
< / span > < span class = "boring" > use juniper::{GraphQLObject, GraphQLUnion};
< / span > < span class = "boring" >
< / span > #[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< S> {
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Human(Human),
Droid(Droid),
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#[from(ignore)]
#[graphql(ignore)]
// ^^^^^^ or `skip`, up to your preference
_State(PhantomData< S> ),
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}
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< span class = "boring" >
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< / span > < span class = "boring" > fn main() {}< / span > < / code > < / pre > < / pre >
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< blockquote >
< p > < strong > WARNING< / strong > : It's the < em > library user's responsibility< / em > to ensure that ignored < a href = "https://doc.rust-lang.org/reference/items/enumerations.html#enumerations" > enum< / a > variant is < strong > never< / strong > returned from resolvers, otherwise resolving the < a href = "https://graphql.org" > GraphQL< / a > query will < strong > panic in runtime< / strong > .< / p >
< / blockquote >
< blockquote >
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< p > < strong > TIP< / strong > : See more available features in the API docs of the < a href = "https://docs.rs/juniper/0.16.0/juniper/derive.GraphQLUnion.html" > < code > #[derive(GraphQLUnion)]< / code > < / a > attribute.< / p >
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< / blockquote >
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