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< h1 id = "eager-loading" > < a class = "header" href = "#eager-loading" > Eager loading< / a > < / h1 >
< 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 >
< 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 >
< blockquote >
< 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 >
< / blockquote >
<!-- TODO: Provide example of solving the problem from "N+1 chapter" once `juniper - eager - loading` support the latest `juniper`. -->
< 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 >
< blockquote >
< 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 >
< p > If you have a GraphQL type like this< / p >
< pre > < code class = "language-graphql" > type User {
id: Int!
country: Country!
}
< / code > < / pre >
< p > You might create the corresponding Rust model type like this:< / p >
< pre > < pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > struct User {
id: i32,
country_id: i32,
}
< span class = "boring" > }< / span > < / code > < / pre > < / pre >
< p > However this approach has one big issue. How are you going to resolve the field < code > User.country< / code >
without doing a database query? All the resolver has access to is a < code > User< / code > with a < code > country_id< / code >
field. It can't get the country without loading it from the database...< / p >
< p > Fundamentally these kinds of model structs don't work for eager loading with GraphQL. So
this library takes a different approach.< / p >
< p > What if we created separate structs for the database models and the GraphQL models? Something
like this:< / p >
< pre > < pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > fn main() {}
< / span > < span class = "boring" >
< / span > mod models {
pub struct User {
id: i32,
country_id: i32
}
pub struct Country {
id: i32,
}
}
struct User {
user: models::User,
country: HasOne< Country> ,
}
struct Country {
country: models::Country
}
enum HasOne< T> {
Loaded(T),
NotLoaded,
}< / code > < / pre > < / pre >
< p > Now we're able to resolve the query with code like this:< / p >
< ol >
< li > Load all the users (first query).< / li >
< li > Map the users to a list of country ids.< / li >
< li > Load all the countries with those ids (second query).< / li >
< li > Pair up the users with the country with the correct id, so change < code > User.country< / code > from
< code > HasOne::NotLoaded< / code > to < code > HasOne::Loaded(matching_country)< / code > .< / li >
< li > When resolving the GraphQL field < code > User.country< / code > simply return the loaded country.< / li >
< / ol >
< h3 id = "a-real-example" > < a class = "header" href = "#a-real-example" > A real example< / a > < / h3 >
< pre > < code class = "language-rust ignore" > use juniper::{Executor, FieldResult};
use juniper_eager_loading::{prelude::*, EagerLoading, HasOne};
use juniper_from_schema::graphql_schema;
use std::error::Error;
// Define our GraphQL schema.
graphql_schema! {
schema {
query: Query
}
type Query {
allUsers: [User!]! @juniper(ownership: "owned")
}
type User {
id: Int!
country: Country!
}
type Country {
id: Int!
}
}
// Our model types.
mod models {
use std::error::Error;
use juniper_eager_loading::LoadFrom;
#[derive(Clone)]
pub struct User {
pub id: i32,
pub country_id: i32
}
#[derive(Clone)]
pub struct Country {
pub id: i32,
}
// This trait is required for eager loading countries.
// It defines how to load a list of countries from a list of ids.
// Notice that `Context` is generic and can be whatever you want.
// It will normally be your Juniper context which would contain
// a database connection.
impl LoadFrom< i32> for Country {
type Error = Box< dyn Error> ;
type Context = super::Context;
fn load(
employments: & [i32],
field_args: & (),
ctx: & Self::Context,
) -> Result< Vec< Self> , Self::Error> {
// ...
< span class = "boring" > unimplemented!()
< / span > }
}
}
// Our sample database connection type.
pub struct DbConnection;
impl DbConnection {
// Function that will load all the users.
fn load_all_users(& self) -> Vec< models::User> {
// ...
< span class = "boring" > unimplemented!()
< / span > }
}
// Our Juniper context type which contains a database connection.
pub struct Context {
db: DbConnection,
}
impl juniper::Context for Context {}
// Our GraphQL user type.
// `#[derive(EagerLoading)]` takes care of generating all the boilerplate code.
#[derive(Clone, EagerLoading)]
// You need to set the context and error type.
#[eager_loading(
context = Context,
error = Box< dyn Error> ,
// These match the default so you wouldn't have to specify them
model = models::User,
id = i32,
root_model_field = user,
)]
pub struct User {
// This user model is used to resolve `User.id`
user: models::User,
// Setup a "has one" association between a user and a country.
//
// We could also have used `#[has_one(default)]` here.
#[has_one(
foreign_key_field = country_id,
root_model_field = country,
graphql_field = country,
)]
country: HasOne< Country> ,
}
// And the GraphQL country type.
#[derive(Clone, EagerLoading)]
#[eager_loading(context = Context, error = Box< dyn Error> )]
pub struct Country {
country: models::Country,
}
// The root query GraphQL type.
pub struct Query;
impl QueryFields for Query {
// The resolver for `Query.allUsers`.
fn field_all_users(
& self,
executor: & Executor< '_, Context> ,
trail: & QueryTrail< '_, User, Walked> ,
) -> FieldResult< Vec< User> > {
let ctx = executor.context();
// Load the model users.
let user_models = ctx.db.load_all_users();
// Turn the model users into GraphQL users.
let mut users = User::from_db_models(& user_models);
// Perform the eager loading.
// `trail` is used to only eager load the fields that are requested. Because
// we're using `QueryTrail`s from "juniper_from_schema" it would be a compile
// error if we eager loaded associations that aren't requested in the query.
User::eager_load_all_children_for_each(& mut users, & user_models, ctx, trail)?;
Ok(users)
}
}
impl UserFields for User {
fn field_id(
& self,
executor: & Executor< '_, Context> ,
) -> FieldResult< & i32> {
Ok(& self.user.id)
}
fn field_country(
& self,
executor: & Executor< '_, Context> ,
trail: & QueryTrail< '_, Country, Walked> ,
) -> FieldResult< & Country> {
// 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 >
< / main >
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