[Postgres] Unable to use #[derive]d custom types with Vec<T>

[jamwaffles]

Bit of a word soup of a title, sorry 😬

I'm trying to deserialize a row into a struct that holds a Vec<Item>, where Item is an enum that derives sqlx::Type. This fails to decode, stating some trait bounds aren't satisfied. Is this a not-yet-implement feature, or am I doing something wrong/missing something in my code?

Code:

use sqlx::postgres::PgQueryAs;

#[derive(
    serde_derive::Deserialize, serde_derive::Serialize, PartialEq, Debug, Eq, Hash, sqlx::Type, Copy, Clone
)]
#[sqlx(rename = "varchar")]
enum Item {
    Foo,
    Bar,
}

#[derive(
    serde_derive::Deserialize, serde_derive::Serialize, sqlx::FromRow, Hash, PartialEq, Eq, Debug, Clone
)]
struct Things {
    items: Vec<Item>,
}

async fn query_things(pool: &sqlx::PgPool) -> Result<Vec<Things>, sqlx::Error> {
    sqlx::query_as("select * from things")
        .fetch_all(&mut pool)
        .await
}

The pile of derives is from our real code. Could any of them be causing issues?

Error:

error[E0277]: the trait bound `std::vec::Vec<Item>: sqlx_core::types::Type<sqlx_core::postgres::database::Postgres>` is not satisfied
   --> src/lib.rs:250:10
    |
250 |         .fetch_all(&mut pool)
    |          ^^^^^^^^^ the trait `sqlx_core::types::Type<sqlx_core::postgres::database::Postgres>` is not implemented for `std::vec::Vec<Item>`
    |
    = help: the following implementations were found:
              <std::vec::Vec<&[u8]> as sqlx_core::types::Type<sqlx_core::postgres::database::Postgres>>
              <std::vec::Vec<&str> as sqlx_core::types::Type<sqlx_core::postgres::database::Postgres>>
              <std::vec::Vec<(T1, T2)> as sqlx_core::types::Type<sqlx_core::postgres::database::Postgres>>
              <std::vec::Vec<(T1, T2, T3)> as sqlx_core::types::Type<sqlx_core::postgres::database::Postgres>>
            and 23 others
    = note: required because of the requirements on the impl of `for<'c> sqlx_core::row::FromRow<'c, sqlx_core::postgres::row::PgRow<'c>>` for `Things`

error[E0277]: the trait bound `[Item]: sqlx_core::types::Type<sqlx_core::postgres::database::Postgres>` is not satisfied
   --> src/lib.rs:250:10
    |
250 |         .fetch_all(&mut pool)
    |          ^^^^^^^^^ the trait `sqlx_core::types::Type<sqlx_core::postgres::database::Postgres>` is not implemented for `[Item]`
    |
    = help: the following implementations were found:
              <[&[u8]] as sqlx_core::types::Type<sqlx_core::postgres::database::Postgres>>
              <[&str] as sqlx_core::types::Type<sqlx_core::postgres::database::Postgres>>
              <[(T1, T2)] as sqlx_core::types::Type<sqlx_core::postgres::database::Postgres>>
              <[(T1, T2, T3)] as sqlx_core::types::Type<sqlx_core::postgres::database::Postgres>>
            and 23 others
    = note: required because of the requirements on the impl of `sqlx_core::decode::Decode<'_, sqlx_core::postgres::database::Postgres>` for `std::vec::Vec<Item>`
    = note: required because of the requirements on the impl of `for<'c> sqlx_core::row::FromRow<'c, sqlx_core::postgres::row::PgRow<'c>>` for `Things`

[psnszsn]

I'm having the same issue.

[janaakhterov]

@jamwaffles I might be misunderstanding, but is your table things a single column of type item[]? From what I can tell it has to be for this to work. If you're your table instead looks like create table things (item Item) then you shouldn't be returning a Vec<Things>, but Vec<Item>.

[jamwaffles]

Sorry for not being clearer in the OP. My table looks like this:

create table my_table (
    id uuid primary_key,
    roles varchar array,
    ...
);

With the matching Rust:

#[derive(
    serde_derive::Deserialize, serde_derive::Serialize, PartialEq, Debug, Eq, Hash, sqlx::Type, Copy, Clone
)]
#[sqlx(rename = "varchar")]
enum Item {
    Foo,
    Bar,
}


#[derive(
    serde_derive::Deserialize, serde_derive::Serialize, sqlx::FromRow, Hash, PartialEq, Eq, Debug, Clone
)]
struct MyThing {
    id: Uuid,

    roles: Vec<Item>,

    // ...
}

Hope that makes things clearer!

[izik1]

Hi! I looked into this as it turns out it would useful for something I need. Unfortunately, it's blocked right now.
The impl itself is fairly simple:

impl<T> Type<Postgres> for Vec<T>
where
    T: Type<Postgres>,
{
    #[inline]
    fn type_info() -> PgTypeInfo {
        <T as Type<Postgres>>::type_info()
    }
}

But this dies because lazy normalization doesn't exist yet, (it tries to recursively resolve the trait forever)

[robo-corg]

@izik1 wouldn't your proposed implementation return type info for T not [T]? I think I have a work around using a newtype but the problem remains that I need a type_info for an array of T.

Using just T, for a custom type called job_stage I get:

thread 'subsystems::storage::sql::tests::test_sql_create_workflow_with_empty_dag_creates_dag' panicked at 'called `Result::unwrap()` on an `Err` value: cannot cast type job_stage to job_stage[]

In 0.3.5 stable.