Select query returns no data - rust

Question

I have the following code:
use sqlx::mysql::*;
mod db;
#[tokio::main]
async fn main() -> Result<(), sqlx::Error> {
println!("Hello, world!");
let pool = MySqlPoolOptions::new()
.max_connections(5)
.connect("connection-string").await?;
#[derive(Debug, PartialEq, Eq)]
struct Room {
name: String
}
let mut rooms: Vec<Room> = vec![];
let mut stream = sqlx::query("SELECT name FROM rooms")
.map(|row: MySqlRow| {
// map the row into a user-defined domain type
//rooms.push( Room { row.name } );
println!("Tester print");
})
.fetch(&pool);
println!("{:?}", rooms);
Ok(())
}
it seems to connect, it doesnt error out, but its not getting any data, or at least the print inside the map function is not getting executed. Anyone know why?

Answer 1

In sqlx, map and fetch are lazy, meaning that they don't do anything unless and until explicitly consumed. This is a very common pattern in Rust and avoids wasting time if you only need part of the results. For your use-case where you want to store (transformed) values into a Vec, the best way to do this is with collect or fetch_all.
collect applies to all kinds of Streams, not just the results from an sqlx query:
let mut rooms = sqlx::query("SELECT name FROM rooms")
.map(|row: MySqlRow| { Room { row.name } })
.fetch(&pool)
.collect::<Result<Vec<_>>>()
.await?;
or fetch_all makes for slightly shorter code but is specific to sqlx (if you look at its source code, it does basically the same thing):
let mut rooms = sqlx::query("SELECT name FROM rooms")
.map(|row: MySqlRow| { Room { row.name } })
.fetch_all(&pool)
.await?;