DataLoader
Seaography uses async_graphql::dataloader in querying nested objects to tackle the N+1 problem.
Consider the following GraphQL query:
{
film(
pagination: { pages: { limit: 3, page: 0 }}
filters: { releaseYear: { gte: "2006" } }
orderBy: { title: ASC }
) {
nodes {
filmId
title
description
releaseYear
filmActor {
actor {
actorId
firstName
lastName
}
}
}
pages
current
}
}
Behind the scene, the following SQL were queried:
SELECT "film"."film_id",
"film"."title",
"film"."description",
"film"."release_year",
"film"."language_id",
"film"."original_language_id",
"film"."rental_duration",
"film"."rental_rate",
"film"."length",
"film"."replacement_cost",
"film"."rating",
"film"."special_features",
"film"."last_update"
FROM "film"
WHERE "film"."release_year" >= '2006'
ORDER BY "film"."title" ASC
LIMIT 3 OFFSET 0
SELECT "film_actor"."actor_id", "film_actor"."film_id", "film_actor"."last_update"
FROM "film_actor"
WHERE "film_actor"."film_id" IN (1, 3, 2)
SELECT "actor"."actor_id", "actor"."first_name", "actor"."last_name", "actor"."last_update"
FROM "actor"
WHERE "actor"."actor_id" IN (24, 162, 20, 160, 1, 188, 123, 30, 53, 40, 2, 64, 85, 198, 10, 19, 108, 90)
Take film_actor
as an example, we want to fetch film_actor
with ID (1, 3, 2)
from the database. We give the ID to DataLoader
, it has two purposes - it tells DataLoader
which rows to be fetched, and, as a unique ID to determine the caller and therefore the proper receiver of the query result.
pub struct FilmActorFK(pub sea_orm::Value);
// film::Model
impl Model {
pub async fn FilmActor<'a>(
&self,
ctx: &async_graphql::Context<'a>,
) -> Option<Vec<super::film_actor::Model>> {
let data_loader = ctx
.data::<async_graphql::dataloader::DataLoader<crate::OrmDataloader>>()
.unwrap();
let from_column: super::film::Column = // ...
let key = FilmActorFK(self.get(from_column));
let data: Option<_> = data_loader.load_one(key) // Batch querying with foreign keys
.await
.unwrap();
data
}
}
Inside the DataLoader
, it will execute the select in batch. Then, return a hashmap with ID as the key. This allow us to associate the query result with the receiver thus return the corresponding result to the proper receiver.
#[async_trait::async_trait]
impl async_graphql::dataloader::Loader<FilmActorFK> for crate::OrmDataloader {
type Value = Vec<super::film_actor::Model>;
type Error = std::sync::Arc<sea_orm::error::DbErr>;
async fn load(
&self,
keys: &[FilmActorFK],
) -> Result<std::collections::HashMap<FilmActorFK, Self::Value>, Self::Error> {
let key_values: Vec<_> = keys
.into_iter()
.map(|key| key.0.to_owned())
.collect();
let to_column: super::film_actor::Column = // ...
let data: std::collections::HashMap<FilmActorFK, Self::Value> = super::film_actor::Entity::find()
.filter(to_column.is_in(key_values)) // Filter by a batch of foreign keys
.all(&self.db)
.await?
.into_iter()
.map(|model| {
let key = FilmActorFK(model.get(to_column));
(key, model) // Collect rows into a hashmap with foreign key as the key
})
.into_group_map();
Ok(data)
}
}