391 lines
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<h1 class="menu-title">Juniper - GraphQL Server for Rust</h1>
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<main>
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<a class="header" href="#interfaces" id="interfaces"><h1>Interfaces</h1></a>
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<p>GraphQL interfaces map well to interfaces known from common object-oriented
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languages such as Java or C#, but Rust has unfortunately not a concept that maps
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perfectly to them. Because of this, defining interfaces in Juniper can require a
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little bit of boilerplate code, but on the other hand gives you full control
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over which type is backing your interface.</p>
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<p>To highlight a couple of different ways you can implement interfaces in Rust,
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let's have a look at the same end-result from a few different implementations:</p>
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<a class="header" href="#traits" id="traits"><h2>Traits</h2></a>
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<p>Traits are maybe the most obvious concept you want to use when building
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interfaces. But because GraphQL supports downcasting while Rust doesn't, you'll
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have to manually specify how to convert a trait into a concrete type. This can
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be done in a couple of different ways:</p>
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<a class="header" href="#downcasting-via-accessor-methods" id="downcasting-via-accessor-methods"><h3>Downcasting via accessor methods</h3></a>
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<pre><pre class="playpen"><code class="language-rust">#[derive(juniper::GraphQLObject)]
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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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#[derive(juniper::GraphQLObject)]
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struct Droid {
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id: String,
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primary_function: String,
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}
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trait Character {
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fn id(&self) -> &str;
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// Downcast methods, each concrete class will need to implement one of these
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fn as_human(&self) -> Option<&Human> { None }
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fn as_droid(&self) -> Option<&Droid> { None }
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}
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impl Character for Human {
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fn id(&self) -> &str { self.id.as_str() }
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fn as_human(&self) -> Option<&Human> { Some(&self) }
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}
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impl Character for Droid {
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fn id(&self) -> &str { self.id.as_str() }
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fn as_droid(&self) -> Option<&Droid> { Some(&self) }
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}
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juniper::graphql_interface!(<'a> &'a Character: () as "Character" where Scalar = <S> |&self| {
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field id() -> &str { self.id() }
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instance_resolvers: |_| {
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// The left hand side indicates the concrete type T, the right hand
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// side should be an expression returning Option<T>
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&Human => self.as_human(),
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&Droid => self.as_droid(),
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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>The <code>instance_resolvers</code> declaration lists all the implementors of the given
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interface and how to resolve them.</p>
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<p>As you can see, you lose a bit of the point with using traits: you need to list
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all the concrete types in the trait itself, and there's a bit of repetition
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going on.</p>
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<a class="header" href="#using-an-extra-database-lookup" id="using-an-extra-database-lookup"><h3>Using an extra database lookup</h3></a>
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<p>If you can afford an extra database lookup when the concrete class is requested,
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you can do away with the downcast methods and use the context instead. Here,
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we'll use two hashmaps, but this could be two tables and some SQL calls instead:</p>
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<pre><pre class="playpen"><code class="language-rust"># use std::collections::HashMap;
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#[derive(juniper::GraphQLObject)]
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#[graphql(Context = Database)]
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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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#[derive(juniper::GraphQLObject)]
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#[graphql(Context = Database)]
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struct Droid {
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id: String,
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primary_function: String,
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}
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struct Database {
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humans: HashMap<String, Human>,
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droids: HashMap<String, Droid>,
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}
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impl juniper::Context for Database {}
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trait Character {
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fn id(&self) -> &str;
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}
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impl Character for Human {
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fn id(&self) -> &str { self.id.as_str() }
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}
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impl Character for Droid {
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fn id(&self) -> &str { self.id.as_str() }
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}
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juniper::graphql_interface!(<'a> &'a Character: Database as "Character" where Scalar = <S> |&self| {
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field id() -> &str { self.id() }
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instance_resolvers: |&context| {
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&Human => context.humans.get(self.id()),
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&Droid => context.droids.get(self.id()),
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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>This removes the need of downcast methods, but still requires some repetition.</p>
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<a class="header" href="#placeholder-objects" id="placeholder-objects"><h2>Placeholder objects</h2></a>
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<p>Continuing on from the last example, the trait itself seems a bit unneccesary.
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Maybe it can just be a struct containing the ID?</p>
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<pre><pre class="playpen"><code class="language-rust"># use std::collections::HashMap;
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#[derive(juniper::GraphQLObject)]
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#[graphql(Context = "Database")]
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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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#[derive(juniper::GraphQLObject)]
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#[graphql(Context = "Database")]
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struct Droid {
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id: String,
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primary_function: String,
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}
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struct Database {
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humans: HashMap<String, Human>,
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droids: HashMap<String, Droid>,
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}
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impl juniper::Context for Database {}
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struct Character {
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id: String,
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}
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juniper::graphql_interface!(Character: Database where Scalar = <S> |&self| {
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field id() -> &str { self.id.as_str() }
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instance_resolvers: |&context| {
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&Human => context.humans.get(&self.id),
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&Droid => context.droids.get(&self.id),
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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>This reduces repetition some more, but might be impractical if the interface's
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surface area is large.</p>
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<a class="header" href="#enums" id="enums"><h2>Enums</h2></a>
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<p>Using enums and pattern matching lies half-way between using traits and using
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placeholder objects. We don't need the extra database call in this case, so
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we'll remove it.</p>
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<pre><pre class="playpen"><code class="language-rust">#[derive(juniper::GraphQLObject)]
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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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#[derive(juniper::GraphQLObject)]
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struct Droid {
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id: String,
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primary_function: String,
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}
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# #[allow(dead_code)]
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enum Character {
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Human(Human),
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Droid(Droid),
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}
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juniper::graphql_interface!(Character: () where Scalar = <S> |&self| {
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field id() -> &str {
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match *self {
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Character::Human(Human { ref id, .. }) |
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Character::Droid(Droid { ref id, .. }) => id,
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}
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}
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instance_resolvers: |_| {
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&Human => match *self { Character::Human(ref h) => Some(h), _ => None },
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&Droid => match *self { Character::Droid(ref d) => Some(d), _ => None },
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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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