How can I get an Axum Handler function to return a Vec? - rust

Question

I'm learning how to use Axum with SQLx starting with this example. The basic example works, but I have problem trying to move forward. I am working with a simple database table as shown below:
todo | description
--------+--------------
todo_1 | doing todo 1
todo_2 | doing todo 2
todo_3 | doing todo 3
I am trying to simply get back "SELECT * FROM todos", but I am getting an error. I think I am getting the return of the Result type wrong but I am not sure what to do next. The entirety of main.rs is shown below.
//! Example of application using <https://github.com/launchbadge/sqlx>
//!
//! Run with
//!
//! ```not_rust
//! cd examples && cargo run -p example-sqlx-postgres
//! ```
//!
//! Test with curl:
//!
//! ```not_rust
//! curl 127.0.0.1:3000
//! curl -X POST 127.0.0.1:3000
//! ```
use axum::{
async_trait,
extract::{Extension, FromRequest, RequestParts},
http::StatusCode,
routing::get,
Router,
};
use sqlx::postgres::{PgPool, PgPoolOptions, PgRow};
use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
use std::{net::SocketAddr, time::Duration};
#[tokio::main]
async fn main() {
tracing_subscriber::registry()
.with(tracing_subscriber::EnvFilter::new(
std::env::var("RUST_LOG").unwrap_or_else(|_| "example_tokio_postgres=debug".into()),
))
.with(tracing_subscriber::fmt::layer())
.init();
let db_connection_str = std::env::var("DATABASE_URL")
.unwrap_or_else(|_| "postgres://postgres:postgres#localhost".to_string());
// setup connection pool
let pool = PgPoolOptions::new()
.max_connections(5)
.connect_timeout(Duration::from_secs(3))
.connect(&db_connection_str)
.await
.expect("can connect to database");
// build our application with some routes
let app = Router::new()
.route(
"/",
get(using_connection_pool_extractor).post(using_connection_extractor),
)
.layer(Extension(pool));
// run it with hyper
let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
tracing::debug!("listening on {}", addr);
axum::Server::bind(&addr)
.serve(app.into_make_service())
.await
.unwrap();
}
// we can extract the connection pool with `Extension`
async fn using_connection_pool_extractor(
Extension(pool): Extension<PgPool>,
) -> Result<Vec<String>, (StatusCode, String)> {
sqlx::query_scalar("select * from todos")
.fetch_one(&pool)
.await
.map_err(internal_error)
}
// we can also write a custom extractor that grabs a connection from the pool
// which setup is appropriate depends on your application
struct DatabaseConnection(sqlx::pool::PoolConnection<sqlx::Postgres>);
#[async_trait]
impl<B> FromRequest<B> for DatabaseConnection
where
B: Send,
{
type Rejection = (StatusCode, String);
async fn from_request(req: &mut RequestParts<B>) -> Result<Self, Self::Rejection> {
let Extension(pool) = Extension::<PgPool>::from_request(req)
.await
.map_err(internal_error)?;
let conn = pool.acquire().await.map_err(internal_error)?;
Ok(Self(conn))
}
}
async fn using_connection_extractor(
DatabaseConnection(conn): DatabaseConnection,
) -> Result<String, (StatusCode, String)> {
let mut conn = conn;
sqlx::query_scalar("select 'hello world from pg'")
.fetch_one(&mut conn)
.await
.map_err(internal_error)
}
/// Utility function for mapping any error into a `500 Internal Server Error`
/// response.
fn internal_error<E>(err: E) -> (StatusCode, String)
where
E: std::error::Error,
{
(StatusCode::INTERNAL_SERVER_ERROR, err.to_string())
}
Compared to the example, I changed this function so that it returns a Vec<String> instead of a plain String, but I get a compiler error:
async fn using_connection_pool_extractor(
Extension(pool): Extension<PgPool>,
) -> Result<Vec<String>, (StatusCode, String)> {
sqlx::query_scalar("select * from todos")
.fetch_one(&pool)
.await
.map_err(internal_error)
}
error[E0277]: the trait bound `fn(Extension<Pool<sqlx::Postgres>>) -> impl Future<Output = Result<Vec<String>, (StatusCode, String)>> {using_connection_pool_extractor}: Handler<_, _>` is not satisfied
--> src/main.rs:52:17
|
52 | get(using_connection_pool_extractor).post(using_connection_extractor),
| --- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Handler<_, _>` is not implemented for `fn(Extension<Pool<sqlx::Postgres>>) -> impl Future<Output = Result<Vec<String>, (StatusCode, String)>> {using_connection_pool_extractor}`
| |
| required by a bound introduced by this call
|
= help: the trait `Handler<T, ReqBody>` is implemented for `axum::handler::Layered<S, T>`
note: required by a bound in `axum::routing::get`
I am not sure what this error is suggesting or if it is even related to the actual problem.

Answer 1

Try using axum::Json:
async fn using_connection_pool_extractor(
Extension(pool): Extension<PgPool>,
) -> Result<axum::Json<Vec<String>>, (StatusCode, String)> {
sqlx::query_scalar("select * from todos")
.fetch_one(&pool)
.await
.map(|todos| axum::Json(todos))
.map_err(internal_error)
}
The reason why is that there's no implementation of the IntoResponse trait for Vec<T>. Here's a longer answer by Axum's author: https://users.rust-lang.org/t/axum-error-handling-trait-question/65530