Loading Data from tables with bytea columns fails due missing FromSqlRow implementation

[MattsSe]

Versions

• Rust:
  nightly-x86_64-apple-darwin (default)
  rustc 1.25.0-nightly (bd98fe0c0 2018-02-06)
• Diesel:
  1.1.1
• Database:
  psql (PostgreSQL) 9.6.7
• Operating System
  MacOs High Sierra

Feature Flags

• diesel: chrono, postgresql

Problem Description

I'm trying to load data from a postgresql table that contains a Bytea column. This fails due missing implementations for FromSqlRow, even if the targeted type doesn't contain a field for that column. Inserting, however works fine. This occurs for all tables that contain at least a bytea column.

What are you trying to accomplish?

Loading data from tables with columns of binary types. How to fetch Binary data?

What is the actual output?

error[E0277]: the trait bound `(i32, std::string::String): diesel::deserialize::FromSqlRow<(diesel::sql_types::Integer, diesel::sql_types::Text, diesel::sql_types::Nullable<diesel::sql_types::Binary>), _>` is not satisfied
  --> diesel-test/src/lib.rs:38:19
   |
38 |     let x = dummy.load::<Dummy>(conn);
   |                   ^^^^ the trait `diesel::deserialize::FromSqlRow<(diesel::sql_types::Integer, diesel::sql_types::Text, diesel::sql_types::Nullable<diesel::sql_types::Binary>), _>` is not implemented for `(i32, std::string::String)`
   |
   = help: the following implementations were found:
             <(A, B) as diesel::deserialize::FromSqlRow<(SA, SB), DB>>
             <(std::collections::Bound<T>, std::collections::Bound<T>) as diesel::deserialize::FromSqlRow<diesel::sql_types::Range<ST>, diesel::pg::Pg>>
   = note: required because of the requirements on the impl of `diesel::Queryable<(diesel::sql_types::Integer, diesel::sql_types::Text, diesel::sql_types::Nullable<diesel::sql_types::Binary>), _>` for `Dummy`
   = note: required because of the requirements on the impl of `diesel::query_dsl::LoadQuery<_, Dummy>` for `dummy::table`

Steps to reproduce

#[macro_use]
extern crate diesel;
#[macro_use]
extern crate diesel_codegen;
extern crate serde;
#[macro_use]
extern crate serde_derive;

use diesel::pg::PgConnection;
table! {
    dummy (id) {
        id -> Int4,
        name -> Varchar,
        image -> Nullable<Bytea>,
    }
}

#[derive(Queryable, Insertable, Debug, Clone)]
#[table_name = "dummy"]
pub struct Dummy {
    pub id: i32,
    pub name: String,
}

fn test(conn: &PgConnection) {
    use dummy::dsl::*;
    use diesel::prelude::*;
    use diesel::*;
    use diesel::dsl::*;
    use diesel::query_dsl::*;

    let insert_value = Dummy {
        id: 5,
        name: "".to_owned(),
    };

    insert_into(dummy).values(&insert_value).execute(conn);

    let result = dummy.load::<Dummy>(conn);
}

[Eijebong]

You're missing the image column in your Dummy struct. Here diesel is telling you that it can't deserialize a (Int4, Varchar, Nullable<Bytea>)into a (i32, String) (which is a tuple representation of your Dummy struct.
Adding a image: Vec<u8> line to your struct should fix this.

[MattsSe]

Got it, thanks. Thought that diesel could omit the nullable field.
Guess I'll use tuple types if I don't need the image data.

[Eijebong]

You could also do that. But if you still want your structure, the common thing is two have two different structs, one that is Insertable and one Queryable.