463 lines
22 KiB
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463 lines
22 KiB
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<main>
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<h1 id="eager-loading"><a class="header" href="#eager-loading">Eager loading</a></h1>
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<p>As a further evolution of the <a href="lookahead.html#n1-problem">dealing with the N+1 problem via look-ahead</a>, we may systematically remodel <a href="https://www.rust-lang.org">Rust</a> types mapping to <a href="https://graphql.org">GraphQL</a> ones in the way to encourage doing eager preloading of data for its <a href="https://spec.graphql.org/October2021#sec-Language.Fields">fields</a> and using the already preloaded data when resolving a particular <a href="https://spec.graphql.org/October2021#sec-Language.Fields">field</a>.</p>
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<p>At the moment, this approach is represented with the <a href="https://docs.rs/juniper-eager-loading"><code>juniper-eager-loading</code></a> crate for <a href="https://docs.rs/juniper">Juniper</a>.</p>
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<blockquote>
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<p><strong>NOTE</strong>: Since this library requires <a href="https://docs.rs/juniper-from-schema"><code>juniper-from-schema</code></a>, it's best first to become familiar with it.</p>
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</blockquote>
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<!-- TODO: Provide example of solving the problem from "N+1 chapter" once `juniper-eager-loading` support the latest `juniper`. -->
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<p>From <a href="https://docs.rs/juniper-eager-loading/latest/juniper_eager_loading#how-this-library-works-at-a-high-level">"How this library works at a high level"</a> and <a href="https://docs.rs/juniper-eager-loading/latest/juniper_eager_loading#a-real-example">"A real example"</a> sections of <a href="https://docs.rs/juniper-eager-loading"><code>juniper-eager-loading</code></a> documentation:</p>
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<blockquote>
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<h3 id="how-this-library-works-at-a-high-level"><a class="header" href="#how-this-library-works-at-a-high-level">How this library works at a high level</a></h3>
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<p>If you have a GraphQL type like this</p>
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<pre><code class="language-graphql">type User {
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id: Int!
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country: Country!
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}
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</code></pre>
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<p>You might create the corresponding Rust model type like this:</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>struct User {
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id: i32,
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country_id: i32,
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}
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<span class="boring">}</span></code></pre></pre>
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<p>However this approach has one big issue. How are you going to resolve the field <code>User.country</code>
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without doing a database query? All the resolver has access to is a <code>User</code> with a <code>country_id</code>
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field. It can't get the country without loading it from the database...</p>
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<p>Fundamentally these kinds of model structs don't work for eager loading with GraphQL. So
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this library takes a different approach.</p>
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<p>What if we created separate structs for the database models and the GraphQL models? Something
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like this:</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">fn main() {}
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</span><span class="boring">
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</span>mod models {
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pub struct User {
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id: i32,
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country_id: i32
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}
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pub struct Country {
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id: i32,
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}
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}
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struct User {
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user: models::User,
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country: HasOne<Country>,
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}
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struct Country {
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country: models::Country
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}
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enum HasOne<T> {
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Loaded(T),
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NotLoaded,
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}</code></pre></pre>
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<p>Now we're able to resolve the query with code like this:</p>
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<ol>
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<li>Load all the users (first query).</li>
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<li>Map the users to a list of country ids.</li>
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<li>Load all the countries with those ids (second query).</li>
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<li>Pair up the users with the country with the correct id, so change <code>User.country</code> from
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<code>HasOne::NotLoaded</code> to <code>HasOne::Loaded(matching_country)</code>.</li>
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<li>When resolving the GraphQL field <code>User.country</code> simply return the loaded country.</li>
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</ol>
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<h3 id="a-real-example"><a class="header" href="#a-real-example">A real example</a></h3>
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<pre><code class="language-rust ignore">use juniper::{Executor, FieldResult};
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use juniper_eager_loading::{prelude::*, EagerLoading, HasOne};
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use juniper_from_schema::graphql_schema;
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use std::error::Error;
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// Define our GraphQL schema.
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graphql_schema! {
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schema {
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query: Query
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}
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type Query {
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allUsers: [User!]! @juniper(ownership: "owned")
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}
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type User {
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id: Int!
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country: Country!
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}
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type Country {
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id: Int!
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}
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}
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// Our model types.
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mod models {
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use std::error::Error;
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use juniper_eager_loading::LoadFrom;
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#[derive(Clone)]
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pub struct User {
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pub id: i32,
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pub country_id: i32
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}
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#[derive(Clone)]
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pub struct Country {
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pub id: i32,
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}
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// This trait is required for eager loading countries.
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// It defines how to load a list of countries from a list of ids.
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// Notice that `Context` is generic and can be whatever you want.
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// It will normally be your Juniper context which would contain
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// a database connection.
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impl LoadFrom<i32> for Country {
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type Error = Box<dyn Error>;
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type Context = super::Context;
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fn load(
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employments: &[i32],
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field_args: &(),
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ctx: &Self::Context,
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) -> Result<Vec<Self>, Self::Error> {
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// ...
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<span class="boring"> unimplemented!()
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</span> }
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}
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}
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// Our sample database connection type.
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pub struct DbConnection;
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impl DbConnection {
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// Function that will load all the users.
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fn load_all_users(&self) -> Vec<models::User> {
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// ...
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<span class="boring"> unimplemented!()
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</span> }
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}
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// Our Juniper context type which contains a database connection.
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pub struct Context {
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db: DbConnection,
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}
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impl juniper::Context for Context {}
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// Our GraphQL user type.
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// `#[derive(EagerLoading)]` takes care of generating all the boilerplate code.
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#[derive(Clone, EagerLoading)]
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// You need to set the context and error type.
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#[eager_loading(
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context = Context,
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error = Box<dyn Error>,
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// These match the default so you wouldn't have to specify them
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model = models::User,
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id = i32,
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root_model_field = user,
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)]
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pub struct User {
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// This user model is used to resolve `User.id`
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user: models::User,
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// Setup a "has one" association between a user and a country.
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//
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// We could also have used `#[has_one(default)]` here.
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#[has_one(
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foreign_key_field = country_id,
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root_model_field = country,
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graphql_field = country,
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)]
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country: HasOne<Country>,
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}
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// And the GraphQL country type.
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#[derive(Clone, EagerLoading)]
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#[eager_loading(context = Context, error = Box<dyn Error>)]
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pub struct Country {
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country: models::Country,
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}
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// The root query GraphQL type.
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pub struct Query;
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impl QueryFields for Query {
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// The resolver for `Query.allUsers`.
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fn field_all_users(
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&self,
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executor: &Executor<'_, Context>,
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trail: &QueryTrail<'_, User, Walked>,
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) -> FieldResult<Vec<User>> {
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let ctx = executor.context();
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// Load the model users.
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let user_models = ctx.db.load_all_users();
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// Turn the model users into GraphQL users.
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let mut users = User::from_db_models(&user_models);
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// Perform the eager loading.
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// `trail` is used to only eager load the fields that are requested. Because
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// we're using `QueryTrail`s from "juniper_from_schema" it would be a compile
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// error if we eager loaded associations that aren't requested in the query.
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User::eager_load_all_children_for_each(&mut users, &user_models, ctx, trail)?;
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Ok(users)
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}
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}
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impl UserFields for User {
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fn field_id(
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&self,
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executor: &Executor<'_, Context>,
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) -> FieldResult<&i32> {
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Ok(&self.user.id)
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}
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fn field_country(
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&self,
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executor: &Executor<'_, Context>,
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trail: &QueryTrail<'_, Country, Walked>,
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) -> FieldResult<&Country> {
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|
// This will unwrap the country from the `HasOne` or return an error if the
|
|
// country wasn't loaded, or wasn't found in the database.
|
|
Ok(self.country.try_unwrap()?)
|
|
}
|
|
}
|
|
|
|
impl CountryFields for Country {
|
|
fn field_id(
|
|
&self,
|
|
executor: &Executor<'_, Context>,
|
|
) -> FieldResult<&i32> {
|
|
Ok(&self.country.id)
|
|
}
|
|
}
|
|
<span class="boring">
|
|
</span><span class="boring">fn main() {}</span></code></pre>
|
|
</blockquote>
|
|
<p>For more details, check out the <a href="https://docs.rs/juniper-eager-loading"><code>juniper-eager-loading</code> documentation</a>.</p>
|
|
<h2 id="full-example"><a class="header" href="#full-example">Full example</a></h2>
|
|
<p>For a full example using eager loading in <a href="https://docs.rs/juniper">Juniper</a> check out the <a href="https://github.com/davidpdrsn/graphql-app-example"><code>davidpdrsn/graphql-app-example</code> repository</a>.</p>
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