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< h1 id = "look-ahead" > < a class = "header" href = "#look-ahead" > Look-ahead< / a > < / h1 >
< blockquote >
< p > In backtracking algorithms, < strong > look ahead< / strong > is the generic term for a subprocedure that attempts to foresee the effects of choosing a branching variable to evaluate one of its values. The two main aims of look-ahead are to choose a variable to evaluate next and to choose the order of values to assign to it.< / p >
< / blockquote >
< p > In < a href = "https://graphql.org" > GraphQL< / a > , look-ahead machinery allows us to introspect the currently < a href = "https://spec.graphql.org/October2021#sec-Execution" > executed< / a > < a href = "https://spec.graphql.org/October2021#sec-Language.Operations%5C" > GraphQL operation< / a > to see which < a href = "https://spec.graphql.org/October2021#sec-Language.Fields" > fields< / a > has been actually selected by it.< / p >
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< p > In < a href = "https://docs.rs/juniper" > Juniper< / a > , it's represented by the < a href = "https://docs.rs/juniper/0.16.0/juniper/executor/struct.Executor.html#method.look_ahead" > < code > Executor::look_ahead()< / code > < / a > method.< / p >
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< pre > < pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > < span class = "boring" > extern crate juniper;
< / span > < span class = "boring" > use juniper::{graphql_object, Executor, GraphQLObject, ScalarValue};
< / span > < span class = "boring" >
< / span > < span class = "boring" > type UserId = i32;
< / span > < span class = "boring" >
< / span > #[derive(GraphQLObject)]
struct Person {
id: UserId,
name: String,
}
struct Query;
#[graphql_object]
// NOTICE: Specifying `ScalarValue` as custom named type parameter,
// so its name is similar to the one used in methods.
#[graphql(scalar = S: ScalarValue)]
impl Query {
fn persons< S: ScalarValue> (executor: & Executor< '_, '_, (), S> ) -> Vec< Person> {
// Let's see which `Person`'s fields were selected in the client query.
for field_name in executor.look_ahead().children().names() {
dbg!(field_name);
}
// ...
< span class = "boring" > unimplemented!()
< / span > }
}
< span class = "boring" > }< / span > < / code > < / pre > < / pre >
< blockquote >
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< p > < strong > TIP< / strong > : < code > S: ScalarValue< / code > type parameter on the method is required here to keep the < a href = "https://docs.rs/juniper/0.16.0/juniper/executor/struct.Executor.html" > < code > Executor< / code > < / a > being generic over < a href = "https://docs.rs/juniper/0.16.0/juniper/trait.ScalarValue.html" > < code > ScalarValue< / code > < / a > types. We, instead, could have used the < a href = "https://docs.rs/juniper/0.16.0/juniper/enum.DefaultScalarValue.html" > < code > DefaultScalarValue< / code > < / a > , which is the default < a href = "https://docs.rs/juniper/0.16.0/juniper/trait.ScalarValue.html" > < code > ScalarValue< / code > < / a > type for the < a href = "https://docs.rs/juniper/0.16.0/juniper/executor/struct.Executor.html" > < code > Executor< / code > < / a > , and make our code more ergonomic, but less flexible and generic.< / p >
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< pre > < pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > < span class = "boring" > extern crate juniper;
< / span > < span class = "boring" > use juniper::{graphql_object, DefaultScalarValue, Executor, GraphQLObject};
< / span > < span class = "boring" >
< / span > < span class = "boring" > type UserId = i32;
< / span > < span class = "boring" >
< / span > < span class = "boring" > #[derive(GraphQLObject)]
< / span > < span class = "boring" > struct Person {
< / span > < span class = "boring" > id: UserId,
< / span > < span class = "boring" > name: String,
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > < span class = "boring" > struct Query;
< / span > < span class = "boring" >
< / span > #[graphql_object]
#[graphql(scalar = DefaultScalarValue)]
impl Query {
fn persons(executor: & Executor< '_, '_, ()> ) -> Vec< Person> {
for field_name in executor.look_ahead().children().names() {
dbg!(field_name);
}
// ...
< span class = "boring" > unimplemented!()
< / span > }
}
< span class = "boring" > }< / span > < / code > < / pre > < / pre >
< / blockquote >
< h2 id = "n1-problem" > < a class = "header" href = "#n1-problem" > N+1 problem< / a > < / h2 >
< p > Naturally, look-ahead machinery allows us to solve < a href = "n_plus_1.html" > the N+1 problem< / a > by introspecting the requested fields and performing loading in batches eagerly, before actual resolving of those fields:< / p >
< pre > < pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > < span class = "boring" > extern crate anyhow;
< / span > < span class = "boring" > extern crate juniper;
< / span > < span class = "boring" > use std::collections::HashMap;
< / span > < span class = "boring" > use anyhow::anyhow;
< / span > < span class = "boring" > use juniper::{graphql_object, Executor, GraphQLObject, ScalarValue};
< / span > < span class = "boring" >
< / span > < span class = "boring" > type CultId = i32;
< / span > < span class = "boring" > type UserId = i32;
< / span > < span class = "boring" >
< / span > < span class = "boring" > struct Repository;
< / span > < span class = "boring" >
< / span > < span class = "boring" > impl juniper::Context for Repository {}
< / span > < span class = "boring" >
< / span > < span class = "boring" > impl Repository {
< / span > < span class = "boring" > async fn load_cult_by_id(& self, cult_id: CultId) -> anyhow::Result< Option< Cult> > { unimplemented!() }
< / span > < span class = "boring" > async fn load_cults_by_ids(& self, cult_ids: & [CultId]) -> anyhow::Result< HashMap< CultId, Cult> > { unimplemented!() }
< / span > < span class = "boring" > async fn load_all_persons(& self) -> anyhow::Result< Vec< Person> > { unimplemented!() }
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > < span class = "boring" > enum Either< L, R> {
< / span > < span class = "boring" > Absent(L),
< / span > < span class = "boring" > Loaded(R),
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > #[derive(Clone, GraphQLObject)]
struct Cult {
id: CultId,
name: String,
}
struct Person {
id: UserId,
name: String,
cult: Either< CultId, Cult> ,
}
#[graphql_object]
#[graphql(context = Repository)]
impl Person {
fn id(& self) -> CultId {
self.id
}
fn name(& self) -> & str {
self.name.as_str()
}
async fn cult(& self, #[graphql(ctx)] repo: & Repository) -> anyhow::Result< Cult> {
match & self.cult {
Either::Loaded(cult) => Ok(cult.clone()),
Either::Absent(cult_id) => {
// Effectively performs the following SQL query:
// SELECT id, name FROM cults WHERE id = ${cult_id} LIMIT 1
repo.load_cult_by_id(*cult_id)
.await?
.ok_or_else(|| anyhow!("No cult exists for ID `{cult_id}`"))
}
}
}
}
struct Query;
#[graphql_object]
#[graphql(context = Repository, scalar = S: ScalarValue)]
impl Query {
async fn persons< S: ScalarValue> (
#[graphql(ctx)] repo: & Repository,
executor: & Executor< '_, '_, Repository, S> ,
) -> anyhow::Result< Vec< Person> > {
// Effectively performs the following SQL query:
// SELECT id, name, cult_id FROM persons
let mut persons = repo.load_all_persons().await?;
// If the `Person.cult` field has been requested.
if executor.look_ahead()
.children()
.iter()
.any(|sel| sel.field_original_name() == "cult")
{
// Gather `Cult.id`s to load eagerly.
let cult_ids = persons
.iter()
.filter_map(|p| {
match & p.cult {
Either::Absent(cult_id) => Some(*cult_id),
// If for some reason a `Cult` is already loaded,
// then just skip it.
Either::Loaded(_) => None,
}
})
.collect::< Vec< _> > ();
// Load the necessary `Cult`s eagerly.
// Effectively performs the following SQL query:
// SELECT id, name FROM cults WHERE id IN (${cult_id1}, ${cult_id2}, ...)
let cults = repo.load_cults_by_ids(& cult_ids).await?;
// Populate `persons` with the loaded `Cult`s, so they do not perform
// any SQL queries on resolving.
for p in & mut persons {
let Either::Absent(cult_id) = & p.cult else { continue; };
p.cult = Either::Loaded(
cults.get(cult_id)
.ok_or_else(|| anyhow!("No cult exists for ID `{cult_id}`"))?
.clone(),
);
}
}
Ok(persons)
}
}
< span class = "boring" > }< / span > < / code > < / pre > < / pre >
< p > And so, performing a < a href = "n_plus_1.html" > GraphQL query which lead to N+1 problem< / a > < / p >
< pre > < code class = "language-graphql" > query {
persons {
id
name
cult {
id
name
}
}
}
< / code > < / pre >
< p > will lead to efficient < a href = "https://en.wikipedia.org/wiki/SQL" > SQL< / a > queries, just as expected:< / p >
< pre > < code class = "language-sql" > SELECT id, name, cult_id FROM persons;
SELECT id, name FROM cults WHERE id IN (1, 2, 3, 4);
< / code > < / pre >
< h2 id = "more-features" > < a class = "header" href = "#more-features" > More features< / a > < / h2 >
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< p > See more available look-ahead features in the API docs of the < a href = "https://docs.rs/juniper/0.16.0/juniper/executor/struct.LookAheadSelection.html" > < code > LookAheadSelection< / code > < / a > and the < a href = "https://docs.rs/juniper/0.16.0/juniper/executor/struct.LookAheadChildren.html" > < code > LookAheadChildren< / code > < / a > .< / p >
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