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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="#unions" id="unions"><h1>Unions</h1></a>
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<p>From a server's point of view, GraphQL unions are similar to interfaces: the
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only exception is that they don't contain fields on their own.</p>
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<p>In Juniper, the <code>graphql_union!</code> has identical syntax to the <a href="interfaces.html">interface
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macro</a>, but does not support defining fields. Therefore, the same
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considerations about using traits, placeholder types, or enums still apply to
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unions.</p>
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<p>If we look at the same examples as in the interfaces chapter, we see the
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similarities and the tradeoffs:</p>
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<a class="header" href="#traits" id="traits"><h2>Traits</h2></a>
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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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// 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 as_human(&self) -> Option<&Human> { Some(&self) }
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}
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impl Character for Droid {
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fn as_droid(&self) -> Option<&Droid> { Some(&self) }
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}
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juniper::graphql_union!(<'a> &'a Character: () as "Character" where Scalar = <S> |&self| {
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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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<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>FIXME: This example does not compile at the moment</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_union!(<'a> &'a Character: Database as "Character" where Scalar = <S> |&self| {
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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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<a class="header" href="#placeholder-objects" id="placeholder-objects"><h2>Placeholder objects</h2></a>
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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_union!(Character: Database where Scalar = <S> |&self| {
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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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<a class="header" href="#enums" id="enums"><h2>Enums</h2></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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# #[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_union!(Character: () where Scalar = <S> |&self| {
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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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