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<ol class="chapter"><li class="affix"><a href="../index.html">Introduction</a></li><li class="affix"><a href="../quickstart.html">Quickstart</a></li><li class="affix"><a href="../types/index.html">Type System</a></li><li><a href="../types/objects/defining_objects.html"><strong aria-hidden="true">1.</strong> Defining objects</a></li><li><ol class="section"><li><a href="../types/objects/complex_fields.html"><strong aria-hidden="true">1.1.</strong> Complex fields</a></li><li><a href="../types/objects/using_contexts.html"><strong aria-hidden="true">1.2.</strong> Using contexts</a></li><li><a href="../types/objects/error_handling.html"><strong aria-hidden="true">1.3.</strong> Error handling</a></li></ol></li><li><a href="../types/other-index.html"><strong aria-hidden="true">2.</strong> Other types</a></li><li><ol class="section"><li><a href="../types/enums.html"><strong aria-hidden="true">2.1.</strong> Enums</a></li><li><a href="../types/interfaces.html" class="active"><strong aria-hidden="true">2.2.</strong> Interfaces</a></li><li><a href="../types/input_objects.html"><strong aria-hidden="true">2.3.</strong> Input objects</a></li><li><a href="../types/scalars.html"><strong aria-hidden="true">2.4.</strong> Scalars</a></li><li><a href="../types/unions.html"><strong aria-hidden="true">2.5.</strong> Unions</a></li></ol></li><li><a href="../schema/schemas_and_mutations.html"><strong aria-hidden="true">3.</strong> Schemas and mutations</a></li><li><a href="../servers/index.html"><strong aria-hidden="true">4.</strong> Adding A Server</a></li><li><ol class="section"><li><a href="../servers/official.html"><strong aria-hidden="true">4.1.</strong> Official Server Integrations</a></li><li><ol class="section"><li><a href="../servers/warp.html"><strong aria-hidden="true">4.1.1.</strong> Warp</a></li><li><a href="../servers/rocket.html"><strong aria-hidden="true">4.1.2.</strong> Rocket</a></li><li><a href="../servers/iron.html"><strong aria-hidden="true">4.1.3.</strong> Iron</a></li><li><a href="../servers/hyper.html"><strong aria-hidden="true">4.1.4.</strong> Hyper</a></li></ol></li><li><a href="../servers/third-party.html"><strong aria-hidden="true">4.2.</strong> Third Party Integrations</a></li></ol></li><li><a href="../advanced/index.html"><strong aria-hidden="true">5.</strong> Advanced Topics</a></li><li><ol class="section"><li><a href="../advanced/introspection.html"><strong aria-hidden="true">5.1.</strong> Introspection</a></li><li><a href="../advanced/non_struct_objects.html"><strong aria-hidden="true">5.2.</strong> Non-struct objects</a></li><li><a href="../advanced/implicit_and_explicit_null.html"><strong aria-hidden="true">5.3.</strong> Implicit and explicit null</a></li><li><a href="../advanced/objects_and_generics.html"><strong aria-hidden="true">5.4.</strong> Objects and generics</a></li><li><a href="../advanced/multiple_ops_per_request.html"><strong aria-hidden="true">5.5.</strong> Multiple operations per request</a></li><li><a href="../advanced/dataloaders.html"><strong aria-hidden="true">5.6.</strong> Dataloaders</a></li><li><a href="../advanced/subscriptions.html"><strong aria-hidden="true">5.7.</strong> Subscriptions</a></li></ol></li></ol>
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<a class="header" href="#interfaces" id="interfaces"><h1>Interfaces</h1></a>
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<p><a href="https://spec.graphql.org/June2018/#sec-Interfaces">GraphQL interfaces</a> map well to interfaces known from common object-oriented languages such as Java or C#, but Rust, unfortunately, has no concept that maps perfectly to them. The nearest analogue of <a href="https://spec.graphql.org/June2018/#sec-Interfaces">GraphQL interfaces</a> are Rust traits, and the main difference is that in GraphQL an <a href="https://spec.graphql.org/June2018/#sec-Interfaces">interface type</a> serves both as an <em>abstraction</em> and a <em>boxed value (downcastable to concrete implementers)</em>, while in Rust, a trait is an <em>abstraction only</em> and <em>to represent such a boxed value a separate type is required</em>, like enum or trait object, because Rust trait doesn't represent a type itself, and so can have no values. This difference imposes some unintuitive and non-obvious corner cases when we try to express <a href="https://spec.graphql.org/June2018/#sec-Interfaces">GraphQL interfaces</a> in Rust, but on the other hand gives you full control over which type is backing your interface, and how it's resolved.</p>
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<p>For implementing <a href="https://spec.graphql.org/June2018/#sec-Interfaces">GraphQL interfaces</a> Juniper provides the <code>#[graphql_interface]</code> macro.</p>
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<a class="header" href="#traits" id="traits"><h2>Traits</h2></a>
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<p>Defining a trait is mandatory for defining a <a href="https://spec.graphql.org/June2018/#sec-Interfaces">GraphQL interface</a>, because this is the <em>obvious</em> way we describe an <em>abstraction</em> in Rust. All <a href="https://spec.graphql.org/June2018/#sec-Interfaces">interface</a> fields are defined as computed ones via trait methods.</p>
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<pre><pre class="playpen"><code class="language-rust"># extern crate juniper;
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use juniper::graphql_interface;
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#[graphql_interface]
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trait Character {
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fn id(&self) -> &str;
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}
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#
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# fn main() {}
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</code></pre></pre>
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<p>However, to return values of such <a href="https://spec.graphql.org/June2018/#sec-Interfaces">interface</a>, we should provide its implementers and the Rust type representing a <em>boxed value of this trait</em>. The last one can be represented in two flavors: enum and <a href="https://doc.rust-lang.org/reference/types/trait-object.html">trait object</a>.</p>
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<a class="header" href="#enum-values-default" id="enum-values-default"><h3>Enum values (default)</h3></a>
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<p>By default, Juniper generates an enum representing the values of the defined <a href="https://spec.graphql.org/June2018/#sec-Interfaces">GraphQL interface</a>, and names it straightforwardly, <code>{Interface}Value</code>.</p>
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<pre><pre class="playpen"><code class="language-rust"># extern crate juniper;
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use juniper::{graphql_interface, GraphQLObject};
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#[graphql_interface(for = [Human, Droid])] // enumerating all implementers is mandatory
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trait Character {
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fn id(&self) -> &str;
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}
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#[derive(GraphQLObject)]
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#[graphql(impl = CharacterValue)] // notice enum name, NOT trait name
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struct Human {
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id: String,
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}
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#[graphql_interface] // implementing requires macro attribute too, (°ｏ°)!
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impl Character for Human {
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fn id(&self) -> &str {
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&self.id
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}
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}
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#[derive(GraphQLObject)]
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#[graphql(impl = CharacterValue)]
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struct Droid {
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id: String,
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}
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#[graphql_interface]
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impl Character for Droid {
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fn id(&self) -> &str {
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&self.id
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}
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}
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# fn main() {
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let human = Human { id: "human-32".to_owned() };
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// Values type for interface has `From` implementations for all its implementers,
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// so we don't need to bother with enum variant names.
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let character: CharacterValue = human.into();
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assert_eq!(character.id(), "human-32");
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# }
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</code></pre></pre>
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<p>Also, enum name can be specified explicitly, if desired.</p>
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<pre><pre class="playpen"><code class="language-rust"># extern crate juniper;
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use juniper::{graphql_interface, GraphQLObject};
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#[graphql_interface(enum = CharaterInterface, for = Human)]
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trait Character {
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fn id(&self) -> &str;
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}
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#[derive(GraphQLObject)]
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#[graphql(impl = CharaterInterface)]
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struct Human {
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id: String,
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home_planet: String,
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}
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#[graphql_interface]
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impl Character for Human {
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fn id(&self) -> &str {
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&self.id
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}
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}
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#
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# fn main() {}
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</code></pre></pre>
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<a class="header" href="#trait-object-values" id="trait-object-values"><h3>Trait object values</h3></a>
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<p>If, for some reason, we would like to use <a href="https://doc.rust-lang.org/reference/types/trait-object.html">trait objects</a> for representing <a href="https://spec.graphql.org/June2018/#sec-Interfaces">interface</a> values incorporating dynamic dispatch, then it should be specified explicitly in the trait definition.</p>
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<p>Downcasting <a href="https://doc.rust-lang.org/reference/types/trait-object.html">trait objects</a> in Rust is not that trivial, that's why macro transforms the trait definition slightly, imposing some additional type parameters under-the-hood.</p>
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<blockquote>
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<p><strong>NOTICE</strong>:<br />
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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/reference/types/trait-object.html">trait object</a> to specify a <a href="https://spec.graphql.org/June2018/#sec-Interfaces">GraphQL interface</a> behind it.</p>
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</blockquote>
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<pre><pre class="playpen"><code class="language-rust"># extern crate juniper;
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# extern crate tokio;
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use juniper::{graphql_interface, GraphQLObject};
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// `dyn` argument accepts the name of type alias for the required trait object,
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// and macro generates this alias automatically.
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#[graphql_interface(dyn = DynCharacter, for = Human)]
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trait Character {
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async fn id(&self) -> &str; // async fields are supported natively
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}
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#[derive(GraphQLObject)]
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#[graphql(impl = DynCharacter<__S>)] // macro adds `ScalarValue` type parameter to trait,
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struct Human { // so it may be specified explicitly when required
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id: String,
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}
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#[graphql_interface(dyn)] // implementing requires to know about dynamic dispatch too
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impl Character for Human {
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async fn id(&self) -> &str {
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&self.id
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}
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}
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#[derive(GraphQLObject)]
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#[graphql(impl = DynCharacter<__S>)]
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struct Droid {
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id: String,
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}
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#[graphql_interface]
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impl Character for Droid {
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async fn id(&self) -> &str {
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&self.id
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}
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}
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# #[tokio::main]
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# async fn main() {
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let human = Human { id: "human-32".to_owned() };
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let character: Box<DynCharacter> = Box::new(human);
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assert_eq!(character.id().await, "human-32");
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# }
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</code></pre></pre>
|
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<a class="header" href="#ignoring-trait-methods" id="ignoring-trait-methods"><h3>Ignoring trait methods</h3></a>
|
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<p>We may want to omit some trait methods to be assumed as <a href="https://spec.graphql.org/June2018/#sec-Interfaces">GraphQL interface</a> fields and ignore them.</p>
|
||
<pre><pre class="playpen"><code class="language-rust"># extern crate juniper;
|
||
use juniper::{graphql_interface, GraphQLObject};
|
||
|
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#[graphql_interface(for = Human)]
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||
trait Character {
|
||
fn id(&self) -> &str;
|
||
|
||
#[graphql(ignore)] // or `#[graphql(skip)]`, your choice
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fn ignored(&self) -> u32 { 0 }
|
||
}
|
||
|
||
#[derive(GraphQLObject)]
|
||
#[graphql(impl = CharacterValue)]
|
||
struct Human {
|
||
id: String,
|
||
}
|
||
#[graphql_interface]
|
||
impl Character for Human {
|
||
fn id(&self) -> &str {
|
||
&self.id
|
||
}
|
||
}
|
||
#
|
||
# fn main() {}
|
||
</code></pre></pre>
|
||
<a class="header" href="#fields-arguments-and-interface-customization" id="fields-arguments-and-interface-customization"><h3>Fields, arguments and interface customization</h3></a>
|
||
<p>Similarly to <a href="https://spec.graphql.org/June2018/#sec-Objects">GraphQL objects</a> Juniper allows to fully customize <a href="https://spec.graphql.org/June2018/#sec-Interfaces">interface</a> fields and their arguments.</p>
|
||
<pre><pre class="playpen"><code class="language-rust"># #![allow(deprecated)]
|
||
# extern crate juniper;
|
||
use juniper::graphql_interface;
|
||
|
||
// Renames the interface in GraphQL schema.
|
||
#[graphql_interface(name = "MyCharacter")]
|
||
// Describes the interface in GraphQL schema.
|
||
#[graphql_interface(description = "My own character.")]
|
||
// Usual Rust docs are supported too as GraphQL interface description,
|
||
// but `description` attribute argument takes precedence over them, if specified.
|
||
/// This doc is absent in GraphQL schema.
|
||
trait Character {
|
||
// Renames the field in GraphQL schema.
|
||
#[graphql(name = "myId")]
|
||
// Deprecates the field in GraphQL schema.
|
||
// Usual Rust `#[deprecated]` attribute is supported too as field deprecation,
|
||
// but `deprecated` attribute argument takes precedence over it, if specified.
|
||
#[graphql(deprecated = "Do not use it.")]
|
||
// Describes the field in GraphQL schema.
|
||
#[graphql(description = "ID of my own character.")]
|
||
// Usual Rust docs are supported too as field description,
|
||
// but `description` attribute argument takes precedence over them, if specified.
|
||
/// This description is absent in GraphQL schema.
|
||
fn id(
|
||
&self,
|
||
// Renames the argument in GraphQL schema.
|
||
#[graphql(name = "myNum")]
|
||
// Describes the argument in GraphQL schema.
|
||
#[graphql(description = "ID number of my own character.")]
|
||
// Specifies the default value for the argument.
|
||
// The concrete value may be omitted, and the `Default::default` one
|
||
// will be used in such case.
|
||
#[graphql(default = 5)]
|
||
num: i32,
|
||
) -> &str;
|
||
}
|
||
#
|
||
# fn main() {}
|
||
</code></pre></pre>
|
||
<a class="header" href="#custom-context" id="custom-context"><h3>Custom context</h3></a>
|
||
<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-Interfaces">GraphQL interface</a> field, specify it as an argument.</p>
|
||
<pre><pre class="playpen"><code class="language-rust"># extern crate juniper;
|
||
# use std::collections::HashMap;
|
||
use juniper::{graphql_interface, GraphQLObject};
|
||
|
||
struct Database {
|
||
humans: HashMap<String, Human>,
|
||
}
|
||
impl juniper::Context for Database {}
|
||
|
||
#[graphql_interface(for = Human)] // look, ma, context type is inferred! ＼(^o^)／
|
||
trait Character { // while still can be specified via `Context = ...` attribute argument
|
||
// If a field argument is named `context` or `ctx`, it's automatically assumed
|
||
// as a context argument.
|
||
fn id(&self, context: &Database) -> Option<&str>;
|
||
|
||
// Otherwise, you may mark it explicitly as a context argument.
|
||
fn name(&self, #[graphql(context)] db: &Database) -> Option<&str>;
|
||
}
|
||
|
||
#[derive(GraphQLObject)]
|
||
#[graphql(impl = CharacterValue, Context = Database)]
|
||
struct Human {
|
||
id: String,
|
||
name: String,
|
||
}
|
||
#[graphql_interface]
|
||
impl Character for Human {
|
||
fn id(&self, db: &Database) -> Option<&str> {
|
||
if db.humans.contains_key(&self.id) {
|
||
Some(&self.id)
|
||
} else {
|
||
None
|
||
}
|
||
}
|
||
|
||
fn name(&self, db: &Database) -> Option<&str> {
|
||
if db.humans.contains_key(&self.id) {
|
||
Some(&self.name)
|
||
} else {
|
||
None
|
||
}
|
||
}
|
||
}
|
||
#
|
||
# fn main() {}
|
||
</code></pre></pre>
|
||
<a class="header" href="#using-executor-and-explicit-generic-scalar" id="using-executor-and-explicit-generic-scalar"><h3>Using executor and explicit generic scalar</h3></a>
|
||
<p>If an <a href="https://docs.rs/juniper/latest/juniper/struct.Executor.html"><code>Executor</code></a> is required in a trait method to resolve a <a href="https://spec.graphql.org/June2018/#sec-Interfaces">GraphQL interface</a> field, specify it as an argument.</p>
|
||
<p>This requires to explicitly parametrize over <a href="https://docs.rs/juniper/latest/juniper/trait.ScalarValue.html"><code>ScalarValue</code></a>, as <a href="https://docs.rs/juniper/latest/juniper/struct.Executor.html"><code>Executor</code></a> does so.</p>
|
||
<pre><pre class="playpen"><code class="language-rust"># extern crate juniper;
|
||
use juniper::{graphql_interface, Executor, GraphQLObject, LookAheadMethods as _, ScalarValue};
|
||
|
||
#[graphql_interface(for = Human, Scalar = S)] // notice specifying `ScalarValue` as existing type parameter
|
||
trait Character<S: ScalarValue> {
|
||
// If a field argument is named `executor`, it's automatically assumed
|
||
// as an executor argument.
|
||
async fn id<'a>(&self, executor: &'a Executor<'_, '_, (), S>) -> &'a str
|
||
where
|
||
S: Send + Sync; // required by `#[async_trait]` transformation ¯\_(ツ)_/¯
|
||
|
||
// Otherwise, you may mark it explicitly as an executor argument.
|
||
async fn name<'b>(
|
||
&'b self,
|
||
#[graphql(executor)] another: &Executor<'_, '_, (), S>,
|
||
) -> &'b str
|
||
where
|
||
S: Send + Sync;
|
||
}
|
||
|
||
#[derive(GraphQLObject)]
|
||
#[graphql(impl = CharacterValue<__S>)]
|
||
struct Human {
|
||
id: String,
|
||
name: String,
|
||
}
|
||
#[graphql_interface(scalar = S)]
|
||
impl<S: ScalarValue> Character<S> for Human {
|
||
async fn id<'a>(&self, executor: &'a Executor<'_, '_, (), S>) -> &'a str
|
||
where
|
||
S: Send + Sync,
|
||
{
|
||
executor.look_ahead().field_name()
|
||
}
|
||
|
||
async fn name<'b>(&'b self, _: &Executor<'_, '_, (), S>) -> &'b str
|
||
where
|
||
S: Send + Sync,
|
||
{
|
||
&self.name
|
||
}
|
||
}
|
||
#
|
||
# fn main() {}
|
||
</code></pre></pre>
|
||
<a class="header" href="#downcasting" id="downcasting"><h3>Downcasting</h3></a>
|
||
<p>By default, the <a href="https://spec.graphql.org/June2018/#sec-Interfaces">GraphQL interface</a> value is downcast to one of its implementer types via matching the enum variant or downcasting the trait object (if <code>dyn</code> macro argument is used).</p>
|
||
<p>However, if some custom logic is needed to downcast a <a href="https://spec.graphql.org/June2018/#sec-Interfaces">GraphQL interface</a> implementer, you may specify either an external function or a trait method to do so.</p>
|
||
<pre><pre class="playpen"><code class="language-rust"># extern crate juniper;
|
||
# use std::collections::HashMap;
|
||
use juniper::{graphql_interface, GraphQLObject};
|
||
|
||
struct Database {
|
||
droids: HashMap<String, Droid>,
|
||
}
|
||
impl juniper::Context for Database {}
|
||
|
||
#[graphql_interface(for = [Human, Droid], context = Database)]
|
||
#[graphql_interface(on Droid = get_droid)] // enables downcasting `Droid` via `get_droid()` function
|
||
trait Character {
|
||
fn id(&self) -> &str;
|
||
|
||
#[graphql(downcast)] // makes method a downcast to `Human`, not a field
|
||
// NOTICE: The method signature may optionally contain `&Database` context argument.
|
||
fn as_human(&self) -> Option<&Human> {
|
||
None
|
||
}
|
||
}
|
||
|
||
#[derive(GraphQLObject)]
|
||
#[graphql(impl = CharacterValue, Context = Database)]
|
||
struct Human {
|
||
id: String,
|
||
}
|
||
#[graphql_interface]
|
||
impl Character for Human {
|
||
fn id(&self) -> &str {
|
||
&self.id
|
||
}
|
||
|
||
fn as_human(&self) -> Option<&Self> {
|
||
Some(self)
|
||
}
|
||
}
|
||
|
||
#[derive(GraphQLObject)]
|
||
#[graphql(impl = CharacterValue, Context = Database)]
|
||
struct Droid {
|
||
id: String,
|
||
}
|
||
#[graphql_interface]
|
||
impl Character for Droid {
|
||
fn id(&self) -> &str {
|
||
&self.id
|
||
}
|
||
}
|
||
|
||
// External downcast 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: &CharacterValue, db: &'db Database) -> Option<&'db Droid> {
|
||
db.droids.get(ch.id())
|
||
}
|
||
#
|
||
# fn main() {}
|
||
</code></pre></pre>
|
||
<p>The attribute syntax <code>#[graphql_interface(on ImplementerType = resolver_fn)]</code> follows the <a href="https://spec.graphql.org/June2018/#example-5cc55">GraphQL syntax for downcasting interface implementer</a>.</p>
|
||
<a class="header" href="#scalarvalue-considerations" id="scalarvalue-considerations"><h2><code>ScalarValue</code> considerations</h2></a>
|
||
<p>By default, <code>#[graphql_interface]</code> macro generates 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-Interfaces">GraphQL interface</a> implementers 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>
|
||
<pre><pre class="playpen"><code class="language-rust"># extern crate juniper;
|
||
use juniper::{graphql_interface, DefaultScalarValue, GraphQLObject};
|
||
|
||
#[graphql_interface(for = [Human, Droid])]
|
||
#[graphql_interface(scalar = DefaultScalarValue)] // removing this line will fail compilation
|
||
trait Character {
|
||
fn id(&self) -> &str;
|
||
}
|
||
|
||
#[derive(GraphQLObject)]
|
||
#[graphql(impl = CharacterValue, Scalar = DefaultScalarValue)]
|
||
struct Human {
|
||
id: String,
|
||
home_planet: String,
|
||
}
|
||
#[graphql_interface(scalar = DefaultScalarValue)]
|
||
impl Character for Human {
|
||
fn id(&self) -> &str {
|
||
&self.id
|
||
}
|
||
}
|
||
|
||
#[derive(GraphQLObject)]
|
||
#[graphql(impl = CharacterValue, Scalar = DefaultScalarValue)]
|
||
struct Droid {
|
||
id: String,
|
||
primary_function: String,
|
||
}
|
||
#[graphql_interface(scalar = DefaultScalarValue)]
|
||
impl Character for Droid {
|
||
fn id(&self) -> &str {
|
||
&self.id
|
||
}
|
||
}
|
||
#
|
||
# fn main() {}
|
||
</code></pre></pre>
|
||
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