Error mapping one column type to an entire struct.

[axos88]

Setup

Versions

• Rust: 1.73.0-nightly
• Diesel: 2.1.0 with diesel-async 0.3.2
• Database: pg 14
• Operating System ubuntu 22.04

Feature Flags

• diesel: postgres, chrono

Problem Description

Can't correctly map one database column to an entire struct and back

Steps to reproduce

diesel::table! {
    users (local, domain) {
        local -> Text,
        domain -> Text,
        password -> Text,
        level -> Nullable<Int4>,
        created -> Nullable<Timestamptz>,
    }
}

#[derive(Queryable, Identifiable, Selectable)]
#[diesel(primary_key(local, domain))]
pub struct User {
    pub local: String,
    pub domain: String,
    pub password: Password,
    pub level: Option<i32>,
    pub created: Option<DateTime<Utc>>,
}

use diesel::{AsExpression, FromSqlRow, Queryable, QueryableByName};
use diesel::backend::Backend;
use diesel::deserialize::FromSql;
use diesel::serialize::{Output, ToSql};
use diesel::sql_types::Text;
use regex::Regex;

#[derive(Clone, Debug, PartialEq, Eq, FromSqlRow, AsExpression, QueryableByName)]
#[sql_type = "Text"]
pub struct Password {
  pub alg: Option<String>,
  pub hash: String
}

impl<DB: Backend> FromSql<String, DB> for Password where String: FromSql<Text, DB> {
  fn from_sql(bytes: DB::RawValue<'_>) -> diesel::deserialize::Result<Self> {
    let re = Regex::new(r"(\{\w+\})?(.*)").expect("Could not compile Password regex");

    let string_values = <String as FromSql<Text, DB>>::from_sql(bytes)?;

    let captures = re.captures(&string_values).expect("Could not capture error");

    let alg = captures.get(0).map(|m| m.as_str().to_string());
    let hash = captures.get(1).map(|m| m.as_str().to_string()).expect("No hash part?");

    Ok(Self { alg, hash })
  }
}

impl<DB: Backend> ToSql<String, DB> for Password where String: ToSql<Text, DB> {
  fn to_sql<'b>(&'b self, out: &mut Output<'b, '_, DB>) -> diesel::serialize::Result {
    let s = format!("{}{}", self.alg.unwrap_or(String::new()), self.hash);
    s.to_sql(out)
  }
}

Expect this to take a db row, and deserialize the password text column into a Password struct correctly, however:

error[E0277]: the trait bound `diesel::expression::select_by::SelectBy<db::model::User, Pg>: CompatibleType<db::model::User, Pg>` is not satisfied
  --> backend/src/routes/api/login.rs:69:8
   |
69 |       .first::<db::User>(&mut conn)
   |        ^^^^^ the trait `CompatibleType<db::model::User, Pg>` is not implemented for `diesel::expression::select_by::SelectBy<db::model::User, Pg>`
   |
   = help: the trait `CompatibleType<U, DB>` is implemented for `diesel::expression::select_by::SelectBy<U, DB>`
   = note: required for `SelectStatement<FromClause<table>, SelectClause<SelectBy<User, Pg>>, NoDistinctClause, ..., ..., ...>` to implement `diesel_async::methods::LoadQuery<'_, _, db::model::User>`
   = note: the full type name has been written to '/home/akos/projects/rust/emailadmin/target/debug/deps/emailadmin-694eaea5ca79c3f9.long-type-3122529722960146015.txt'

Checklist

• I have already looked over the issue tracker and the discussion forum for similar possible closed issues.

• This issue can be reproduced on Rust's stable channel. (Your issue will be
  closed if this is not the case)
• This issue can be reproduced without requiring a third party crate

[axos88]

Probably not a real bug, but the error message is too cryptic to be of any use, and I could not find any examples in the docs on how to achieve this.

[axos88]

Turns out the issue is in the FromSql implementation which should be FromSql<Text, DB>.

I'll keep this open in case there is a possibility to add a more helpful error message.

[weiznich]

This seems to be more a support question, which would belong to the discussion forum.

For the error messages: There is already #151 to track that, so even for that it's a duplicated issue. In addition there are flags for this specific case to improve the generated error messages. They are documented here and here

Closed as duplicate/misplaced support question in our issue tracker.