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

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.

score 4 · Answer 1 · edited Sep 18 '22 at 03:50

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

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

1 Answers1