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<main>
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<h1 id="context"><a class="header" href="#context">Context</a></h1>
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<p><em>Context</em> is a feature in <a href="https://docs.rs/juniper">Juniper</a> that lets <a href="https://spec.graphql.org/October2021#sec-Language.Fields">field</a> resolvers access global data, most commonly database connections or authentication information.</p>
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<p>Let's say that we have a simple <code>User</code>s database in a <code>HashMap</code>:</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(dead_code)]
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</span><span class="boring">use std::collections::HashMap;
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</span><span class="boring">
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</span>struct Database {
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users: HashMap<i32, User>,
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}
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struct User {
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id: i32,
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name: String,
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friend_ids: Vec<i32>,
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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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<p>We would like to define a <code>friends</code> <a href="https://spec.graphql.org/October2021#sec-Language.Fields">field</a> on <code>User</code> that returns a list of <code>User</code> <a href="https://spec.graphql.org/October2021#sec-Objects">objects</a>. In order to write such a <a href="https://spec.graphql.org/October2021#sec-Language.Fields">field</a> we need to query a <code>Database</code>. To accomplish this we must first mark the <code>Database</code> as a valid context type and then assign it to the <code>User</code> <a href="https://spec.graphql.org/October2021#sec-Objects">object</a>. To gain access to the context in the <code>friends</code> <a href="https://spec.graphql.org/October2021#sec-Language.Fields">field</a>, we need to specify an argument with the same type as the specified context:</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 std::collections::HashMap;
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</span><span class="boring">use juniper::graphql_object;
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</span><span class="boring">
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</span>struct Database {
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users: HashMap<i32, User>,
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}
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// Mark the `Database` as a valid context type for Juniper.
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impl juniper::Context for Database {}
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struct User {
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id: i32,
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name: String,
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friend_ids: Vec<i32>,
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}
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#[graphql_object]
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#[graphql(context = Database)] // assign `Database` as the context type
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impl User {
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// Inject the `Database` context by specifying an argument with the
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// context type:
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// - the type must be a reference;
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// - the name of the argument SHOULD be `context` (or `ctx`).
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fn friends<'db>(&self, context: &'db Database) -> Vec<&'db User> {
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// ^^^^^^^ or `ctx`, up to your preference
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self.friend_ids.iter()
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.map(|id| {
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context.users.get(&id).expect("could not find `User` with ID")
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})
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.collect()
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}
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fn friend<'db>(
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&self,
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id: i32,
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// Alternatively, the context argument may be marked with an attribute,
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// and thus, named arbitrary.
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#[graphql(context)] db: &'db Database,
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// ^^^^^^^ or `ctx`, up to your preference
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) -> Option<&'db User> {
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self.friend_ids.contains(&id).then(|| {
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db.users.get(&id).expect("could not find `User` with ID")
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})
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}
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fn name(&self) -> &str {
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self.name.as_str()
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}
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fn id(&self) -> i32 {
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self.id
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}
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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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<h3 id="mutating-and-mutable-references"><a class="header" href="#mutating-and-mutable-references">Mutating and mutable references</a></h3>
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<p>Context cannot be a mutable reference as <a href="https://spec.graphql.org/October2021#sec-Language.Fields">fields</a> may be resolved concurrently. If something in the context requires a mutable reference, the context type should leverage the <a href="https://doc.rust-lang.org/reference/interior-mutability.html#interior-mutability"><em>interior mutability</em> pattern</a> (e.g. use <code>RwLock</code>, <code>RefCell</code> or similar).</p>
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<p>For example, when using async runtime with <a href="https://en.wikipedia.org/wiki/Work_stealing">work stealing</a> (like <a href="https://docs.rs/tokio"><code>tokio</code></a>), which obviously requires thread safety in addition, we will need to use a corresponding async version of <code>RwLock</code>:</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">extern crate tokio;
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</span><span class="boring">use std::collections::HashMap;
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</span><span class="boring">use juniper::graphql_object;
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</span>use tokio::sync::RwLock;
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struct Database {
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requested_count: HashMap<i32, i32>,
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}
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// Since we cannot directly implement `juniper::Context`
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// for `RwLock`, we use the newtype idiom.
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struct DatabaseContext(RwLock<Database>);
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impl juniper::Context for DatabaseContext {}
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struct User {
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id: i32,
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name: String
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}
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#[graphql_object]
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#[graphql(context = DatabaseContext)]
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impl User {
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async fn times_requested<'db>(&self, ctx: &'db DatabaseContext) -> i32 {
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// Acquire a mutable reference and `.await` if async `RwLock` is used,
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// which is necessary if context consists of async operations like
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// querying remote databases.
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// Obtain base type.
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let DatabaseContext(db) = ctx;
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// If context is immutable use `.read()` on `RwLock` instead.
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let mut db = db.write().await;
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// Perform a mutable operation.
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db.requested_count
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.entry(self.id)
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.and_modify(|e| *e += 1)
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.or_insert(1)
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.clone()
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}
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fn name(&self) -> &str {
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self.name.as_str()
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}
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|
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fn id(&self) -> i32 {
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self.id
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}
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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>
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<p><strong>TIP</strong>: Replace <code>tokio::sync::RwLock</code> with <code>std::sync::RwLock</code> (or similar) if you don't intend to use async resolving.</p>
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</blockquote>
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