How to define a Vec of Enums as a field in rust sqlx model

Question

I am trying to load an array field in Postgres to a Rust struct as follows

use sqlx::{Pool, PgConnection, PgPool, FromRow};
use sqlx::postgres::PgQueryAs;

#[derive(Copy, Clone, sqlx::Type)]
#[sqlx(rename = "VARCHAR")]
#[sqlx(rename_all = "snake_case")]
enum Coupon {
    ChristmasSaleCoupon,
    NewYearSaleCoupon,
}

#[derive(FromRow, Clone)]
struct UserCouponMap {
    pub id: i64,
    pub user_id: i64,
    pub coupons: Vec<Coupon>,
}

impl UserCouponMap {
    pub async fn get_for_userid(db_pool: Pool<PgConnection>, user_id: i64) -> Vec<UserCouponMap> {
        let user_coupon_map: Vec<UserCouponMap> = sqlx::query_as("SELECT * FROM user_coupon_map WHERE user_id = $1")
            .bind(user_id)
            .fetch_all(db_pool)
            .await
            .expect("failed to fetch user coupon map");
        user_coupon_map
    }
}


#[tokio::main]
async fn fetch_coupons_for_user_id(user_id: i64) {
    let pool = PgPool::new("postgresql://asnim@dbhost:5732").await.expect("expected unwrap to succeed");
    let user_coupon_map = UserCouponMap::get_for_userid(pool, user_id).await;
}

fn main() {
    fetch_coupons_for_user_id(20);
}

As per the Array documentation, I have implemented sqlx::Type for Coupon.

Still, the compiler says, some trait is not satisfied.

   Compiling playground v0.1.0 (/Users/asnimansari/CLionProjects/playground)
error[E0277]: the trait bound `Vec<Coupon>: Type<Postgres>` is not satisfied
  --> src/main.rs:23:14
   |
23 |             .fetch_all(db_pool)
   |              ^^^^^^^^^ the trait `Type<Postgres>` is not implemented for `Vec<Coupon>`
   |
   = help: the following implementations were found:
             <Vec<&[u8]> as Type<Postgres>>
             <Vec<&str> as Type<Postgres>>
             <Vec<(T1, T2)> as Type<Postgres>>
             <Vec<(T1, T2, T3)> as Type<Postgres>>
           and 29 others
   = note: required because of the requirements on the impl of `for<'c> FromRow<'c, PgRow<'c>>` for `UserCouponMap`

error[E0277]: the trait bound `[Coupon]: Type<Postgres>` is not satisfied
  --> src/main.rs:23:14
   |
23 |             .fetch_all(db_pool)
   |              ^^^^^^^^^ the trait `Type<Postgres>` is not implemented for `[Coupon]`
   |
   = help: the following implementations were found:
             <[&[u8]] as Type<Postgres>>
             <[&str] as Type<Postgres>>
             <[(T1, T2)] as Type<Postgres>>
             <[(T1, T2, T3)] as Type<Postgres>>
           and 29 others
   = note: required because of the requirements on the impl of `sqlx::decode::Decode<'_, Postgres>` for `Vec<Coupon>`
   = note: required because of the requirements on the impl of `for<'c> FromRow<'c, PgRow<'c>>` for `UserCouponMap`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

What am I missing here?

My Cargo file has the following dependencies

sqlx = { version = "0.3.5", default-features = false, features = ["runtime-tokio", "macros", "postgres", "all-type"] }
tokio = { version = "0.2.21", features = ["full"] }

Answer 1

One way it should work is to use explicit column list and declare rust type in the query. For OP case:

       let user_coupon_map: Vec<UserCouponMap> = sqlx::query_as!(r#"SELECT id, user_id, coupons as "coupons: Vec<Coupon>" FROM user_coupon_map WHERE user_id = $1"#)...

It may not cover all your needs. If you use query builder like sea_query, then it won't do.