How do I perform a delete with sub-query in Diesel against a Postgres database?

Question

I have the following schema in a Postgres database:

Table A {
  ID
  Name
}

Table B {
  ID FOREIGN KEY (A.ID)
}

I'm trying to write the following query in Diesel:

DELETE FROM B
WHERE B.ID in (SELECT ID from A WHERE A.Name = $VAR)

$VAR is a variable passed by my application.

My first attempt to write this was the following:

fn deleteB(conn: &PgConnection, n: &str) {
    use schema::A::dsl::*;
    use schema::A;
    use schema::B;

    let res = A::table
        .filter(Name.eq(n))
        .select(ID);
        .load(conn)
        .unwrap();
    assert!(res.len() < 2);
    let b_id: i32 = *res.iter().nth(1).unwrap_or(&0);

    let _rows = diesel::delete(
        B::table
        .filter(ID.eq(n_id))
    )
    .execute(conn)
    .unwrap();
}

This compiles but it doesn't work: the SELECT statement for the ID always returns 0. It doesn't match any of the inserted records in A even though I checked by hand they are there. I'm sure I have an error in the way the match is being done (&str vs &String maybe?), but I decided to try a different solution since I didn't like this one anyway since it has to execute two separate statements against the database.

My second attempt looks like this:

fn deleteB(conn: &PgConnection, n: &str) {
    use schema::A::dsl::*;
    use schema::A;
    use schema::B;

    let source = B::table.inner_join(A::table)
            .filter(Name.eq(n));
    let s = delete(source)
            .execute(conn)
            .unwrap();
}

This looks more Diesel-idiomatic to me (spoiler alert, I barely know the framework) but sure enough, it doesn't compile:

    |
410 |     let s = delete(source)
    |                    ^^^^^^ the trait `diesel::query_builder::IntoUpdateTarget` is not implemented for `diesel::query_builder::SelectStatement<diesel::query_source::joins::JoinOn<diesel::query_source::joins::Join<mobility2::schema::B::table...` (very long error)

I was looking into the aforementioned trait when I realised this seems like a trivial operation (the delete itself) and I would probably be better off asking for help.

How do I properly write the idiomatic Diesel code to execute the delete statement I want to execute?

Answer 1

As Diesel statements map quite literally to SQL, the query you have written would result in:

DELETE FROM TABLE B INNER JOIN TABLE A ON … WHERE name = $1

That's not valid SQL, so it results in a compile time error.

To get the query you want to write, you need to do something like:

#[macro_use]
extern crate diesel;

use diesel::delete;
use diesel::prelude::*;

mod schema {
    table! {
        A(ID) {
            ID -> Integer,
            name -> Text,
        }
    }

    table! {
        B(ID) {
            ID -> Integer,
        }
    }

    allow_tables_to_appear_in_same_query!(A, B);
}

fn deleteB(conn: &PgConnection, n: &str) {
    use schema::A;
    use schema::B;

    let s = delete(B::table)
        .filter(B::ID.eq_any(A::table.filter(A::name.eq(n)).select(A::ID)))
        .execute(conn)
        .unwrap();
}