MultiConnection not usable for queries containing `Selectable` or boxed queries. #3566

weiznich · 2023-03-25T16:14:45Z

Setup

Versions

Rust:
Diesel: e548447
Database:
Operating System

Feature Flags

diesel: sqlite postgres

Problem Description

What are you trying to accomplish?

I've tried the following code:

use diesel::prelude::*;

table! {
    users {
        id -> Integer,
        name -> Text,
    }
}

#[derive(Selectable, Queryable)]
struct User {
    id: i32,
}

#[derive(diesel::MultiConnection)]
enum DbConnection {
    Sqlite(SqliteConnection),
    Pg(PgConnection)
}

fn main() {
    let conn = &mut DbConnection::establish("…").unwrap();

    let u = users::table.into_boxed().load::<(i32, String)>(conn);

    let u = users::table
        .select(User::as_select())
        .load(conn);
}

What is the expected output?

Code compiles

What is the actual output?

Code does not compile:

error[E0277]: the trait bound `BoxedSelectStatement<'_, (diesel::sql_types::Integer, diesel::sql_types::Text), FromClause<users::table>, multi_connection_impl::backend::MultiBackend>: QueryFragment<Sqlite>` is not satisfied
    --> src/main.rs:24:61
     |
24   |     let u = users::table.into_boxed().load::<(i32, String)>(conn);
     |                                       ----                  ^^^^ the trait `QueryFragment<Sqlite>` is not implemented for `BoxedSelectStatement<'_, (diesel::sql_types::Integer, diesel::sql_types::Text), FromClause<users::table>, multi_connection_impl::backend::MultiBackend>`
     |                                       |
     |                                       required by a bound introduced by this call
     |
     = help: the following other types implement trait `QueryFragment<DB, SP>`:
               <BoxedSelectStatement<'a, ST, QS, DB, GB> as QueryFragment<DB, AnsiSqlSelectStatement>>
               <BoxedSelectStatement<'a, ST, QS, DB, GB> as QueryFragment<DB>>
               <BoxedSelectStatement<'a, ST, QS, multi_connection_impl::backend::MultiBackend, GB> as QueryFragment<multi_connection_impl::backend::MultiBackend, MultiSelectStatementSyntax>>
note: required for `BoxedSelectStatement<'_, (Integer, Text), FromClause<table>, MultiBackend>` to implement `QueryFragment<multi_connection_impl::backend::MultiBackend, MultiSelectStatementSyntax>`
    --> src/main.rs:15:10
     |
15   | #[derive(diesel::MultiConnection)]
     |          ^^^^^^^^^^^^^^^^^^^^^^^ unsatisfied trait bound introduced in this `derive` macro
     = note: the full type name has been written to '/tmp/diesel_test/target/debug/deps/diesel_test-e9839cd4beffff40.long-type-6862964700214725501.txt'
     = note: 1 redundant requirement hidden
     = note: required for `BoxedSelectStatement<'_, (Integer, Text), FromClause<table>, MultiBackend>` to implement `QueryFragment<multi_connection_impl::backend::MultiBackend>`
     = note: the full type name has been written to '/tmp/diesel_test/target/debug/deps/diesel_test-e9839cd4beffff40.long-type-17859806514065189679.txt'
     = note: required for `BoxedSelectStatement<'_, (Integer, Text), FromClause<table>, MultiBackend>` to implement `LoadQuery<'_, DbConnection, (i32, String)>`
     = note: the full type name has been written to '/tmp/diesel_test/target/debug/deps/diesel_test-e9839cd4beffff40.long-type-17859806514065189679.txt'
note: required by a bound in `diesel::RunQueryDsl::load`
    --> /home/weiznich/Documents/rust/diesel/diesel/src/query_dsl/mod.rs:1543:15
     |
1543 |         Self: LoadQuery<'query, Conn, U>,
     |               ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::load`
     = note: this error originates in the derive macro `diesel::MultiConnection` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the trait bound `BoxedSelectStatement<'_, (diesel::sql_types::Integer, diesel::sql_types::Text), FromClause<users::table>, multi_connection_impl::backend::MultiBackend>: QueryFragment<Pg>` is not satisfied
    --> src/main.rs:24:61
     |
24   |     let u = users::table.into_boxed().load::<(i32, String)>(conn);
     |                                       ----                  ^^^^ the trait `QueryFragment<Pg>` is not implemented for `BoxedSelectStatement<'_, (diesel::sql_types::Integer, diesel::sql_types::Text), FromClause<users::table>, multi_connection_impl::backend::MultiBackend>`
     |                                       |
     |                                       required by a bound introduced by this call
     |
     = help: the following other types implement trait `QueryFragment<DB, SP>`:
               <BoxedSelectStatement<'a, ST, QS, DB, GB> as QueryFragment<DB, AnsiSqlSelectStatement>>
               <BoxedSelectStatement<'a, ST, QS, DB, GB> as QueryFragment<DB>>
               <BoxedSelectStatement<'a, ST, QS, multi_connection_impl::backend::MultiBackend, GB> as QueryFragment<multi_connection_impl::backend::MultiBackend, MultiSelectStatementSyntax>>
note: required for `BoxedSelectStatement<'_, (Integer, Text), FromClause<table>, MultiBackend>` to implement `QueryFragment<multi_connection_impl::backend::MultiBackend, MultiSelectStatementSyntax>`
    --> src/main.rs:15:10
     |
15   | #[derive(diesel::MultiConnection)]
     |          ^^^^^^^^^^^^^^^^^^^^^^^ unsatisfied trait bound introduced in this `derive` macro
     = note: the full type name has been written to '/tmp/diesel_test/target/debug/deps/diesel_test-e9839cd4beffff40.long-type-6862964700214725501.txt'
     = note: 1 redundant requirement hidden
     = note: required for `BoxedSelectStatement<'_, (Integer, Text), FromClause<table>, MultiBackend>` to implement `QueryFragment<multi_connection_impl::backend::MultiBackend>`
     = note: the full type name has been written to '/tmp/diesel_test/target/debug/deps/diesel_test-e9839cd4beffff40.long-type-17859806514065189679.txt'
     = note: required for `BoxedSelectStatement<'_, (Integer, Text), FromClause<table>, MultiBackend>` to implement `LoadQuery<'_, DbConnection, (i32, String)>`
     = note: the full type name has been written to '/tmp/diesel_test/target/debug/deps/diesel_test-e9839cd4beffff40.long-type-17859806514065189679.txt'
note: required by a bound in `diesel::RunQueryDsl::load`
    --> /home/weiznich/Documents/rust/diesel/diesel/src/query_dsl/mod.rs:1543:15
     |
1543 |         Self: LoadQuery<'query, Conn, U>,
     |               ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::load`
     = note: this error originates in the derive macro `diesel::MultiConnection` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the trait bound `LimitOffsetClause<diesel::query_builder::limit_clause::NoLimitClause, diesel::query_builder::offset_clause::NoOffsetClause>: IntoBoxedClause<'_, multi_connection_impl::backend::MultiBackend>` is not satisfied
  --> src/main.rs:24:26
   |
24 |     let u = users::table.into_boxed().load::<(i32, String)>(conn);
   |                          ^^^^^^^^^^ the trait `IntoBoxedClause<'_, multi_connection_impl::backend::MultiBackend>` is not implemented for `LimitOffsetClause<diesel::query_builder::limit_clause::NoLimitClause, diesel::query_builder::offset_clause::NoOffsetClause>`
   |
   = help: the trait `IntoBoxedClause<'a, Sqlite>` is implemented for `LimitOffsetClause<diesel::query_builder::limit_clause::NoLimitClause, diesel::query_builder::offset_clause::NoOffsetClause>`
   = note: required for `SelectStatement<FromClause<users::table>>` to implement `BoxedDsl<'_, multi_connection_impl::backend::MultiBackend>`

error[E0277]: the trait bound `LimitOffsetClause<diesel::query_builder::limit_clause::NoLimitClause, diesel::query_builder::offset_clause::NoOffsetClause>: IntoBoxedClause<'_, multi_connection_impl::backend::MultiBackend>` is not satisfied
    --> src/main.rs:24:26
     |
24   |     let u = users::table.into_boxed().load::<(i32, String)>(conn);
     |                          ^^^^^^^^^^ the trait `IntoBoxedClause<'_, multi_connection_impl::backend::MultiBackend>` is not implemented for `LimitOffsetClause<diesel::query_builder::limit_clause::NoLimitClause, diesel::query_builder::offset_clause::NoOffsetClause>`
     |
     = help: the trait `IntoBoxedClause<'a, Sqlite>` is implemented for `LimitOffsetClause<diesel::query_builder::limit_clause::NoLimitClause, diesel::query_builder::offset_clause::NoOffsetClause>`
     = note: required for `SelectStatement<FromClause<users::table>>` to implement `BoxedDsl<'_, multi_connection_impl::backend::MultiBackend>`
     = note: 1 redundant requirement hidden
     = note: required for `users::table` to implement `BoxedDsl<'_, multi_connection_impl::backend::MultiBackend>`
note: required by a bound in `diesel::QueryDsl::into_boxed`
    --> /home/weiznich/Documents/rust/diesel/diesel/src/query_dsl/mod.rs:1299:15
     |
1299 |         Self: methods::BoxedDsl<'a, DB>,
     |               ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `QueryDsl::into_boxed`

error[E0277]: the trait bound `SelectBy<User, multi_connection_impl::backend::MultiBackend>: QueryFragment<Sqlite>` is not satisfied
    --> src/main.rs:28:15
     |
28   |         .load(conn);
     |          ---- ^^^^ the trait `QueryFragment<Sqlite>` is not implemented for `SelectBy<User, multi_connection_impl::backend::MultiBackend>`
     |          |
     |          required by a bound introduced by this call
     |
     = help: the trait `QueryFragment<DB>` is implemented for `SelectBy<T, DB>`
     = note: required for `diesel::query_builder::select_clause::SelectClause<SelectBy<User, multi_connection_impl::backend::MultiBackend>>` to implement `QueryFragment<Sqlite>`
     = note: 4 redundant requirements hidden
     = note: required for `SelectStatement<FromClause<table>, SelectClause<SelectBy<User, MultiBackend>>>` to implement `QueryFragment<multi_connection_impl::backend::MultiBackend>`
     = note: the full type name has been written to '/tmp/diesel_test/target/debug/deps/diesel_test-e9839cd4beffff40.long-type-13019818345722010715.txt'
     = note: required for `SelectStatement<FromClause<table>, SelectClause<SelectBy<User, MultiBackend>>>` to implement `LoadQuery<'_, DbConnection, User>`
     = note: the full type name has been written to '/tmp/diesel_test/target/debug/deps/diesel_test-e9839cd4beffff40.long-type-13019818345722010715.txt'
note: required by a bound in `diesel::RunQueryDsl::load`
    --> /home/weiznich/Documents/rust/diesel/diesel/src/query_dsl/mod.rs:1543:15
     |
1543 |         Self: LoadQuery<'query, Conn, U>,
     |               ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::load`

error[E0277]: the trait bound `SelectBy<User, multi_connection_impl::backend::MultiBackend>: QueryFragment<Pg>` is not satisfied
    --> src/main.rs:28:15
     |
28   |         .load(conn);
     |          ---- ^^^^ the trait `QueryFragment<Pg>` is not implemented for `SelectBy<User, multi_connection_impl::backend::MultiBackend>`
     |          |
     |          required by a bound introduced by this call
     |
     = help: the trait `QueryFragment<DB>` is implemented for `SelectBy<T, DB>`
     = note: required for `diesel::query_builder::select_clause::SelectClause<SelectBy<User, multi_connection_impl::backend::MultiBackend>>` to implement `QueryFragment<Pg>`
     = note: 4 redundant requirements hidden
     = note: required for `SelectStatement<FromClause<table>, SelectClause<SelectBy<User, MultiBackend>>>` to implement `QueryFragment<multi_connection_impl::backend::MultiBackend>`
     = note: the full type name has been written to '/tmp/diesel_test/target/debug/deps/diesel_test-e9839cd4beffff40.long-type-13019818345722010715.txt'
     = note: required for `SelectStatement<FromClause<table>, SelectClause<SelectBy<User, MultiBackend>>>` to implement `LoadQuery<'_, DbConnection, User>`
     = note: the full type name has been written to '/tmp/diesel_test/target/debug/deps/diesel_test-e9839cd4beffff40.long-type-13019818345722010715.txt'
note: required by a bound in `diesel::RunQueryDsl::load`
    --> /home/weiznich/Documents/rust/diesel/diesel/src/query_dsl/mod.rs:1543:15
     |
1543 |         Self: LoadQuery<'query, Conn, U>,
     |               ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::load`

(That sounds that some queryfragment impls in diesel are to restrictive for using selectable and/or boxed queries with the new MultiConnection derive.

Are you seeing any additional errors?

Checklist

I have already looked over the issue tracker and the disussion 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

The text was updated successfully, but these errors were encountered:

weiznich · 2023-03-31T12:23:29Z

I did some work on this and it seems like that the problem with SelectBy is rather easy to solve by introducing the following trait:

    pub trait CompatibleBackend<DB>: Backend {}
    impl<DB> CompatibleBackend<DB> for DB where DB: Backend {}

then we can provide the corresponding additional impl for the generated multibackend type and restrict the QueryFragment impl for SelectBy on that.

Unfortunately that will not work for the boxed select statement case, due to the fact that a boxed select statement internally stores boxed query fragment trait objects for a specific backend type and we cannot simply convert them to something else. At that point I'm not even sure if that ever can be supported :(

This commit fixes various issues with backend dependent `QueryFragment` impls in combination with `#[derive(Multiconnection)]`. In all cases we solve these issues by not calling the concrete backend specific `QueryFragment` impl early in the query building process. Instead we call the generic `MultiBackend` specific impl, that later defer to the correct implementation specific one. In addition this commit adds some test for the fixed cases.

This commit fixes various issues with backend dependent `QueryFragment` impls in combination with `#[derive(Multiconnection)]`. In all cases we solve these issues by not calling the concrete backend specific `QueryFragment` impl early in the query building process. Instead we call the generic `MultiBackend` specific impl, that later defer to the correct implementation specific one. In addition this commit adds some test for the fixed cases. Acked-by: Georg Semmler <github@weiznich.de>

Fix #3566

weiznich added bug hole in api internal or unreleased api Release Blocker This issue needs to be solved before we can release a new version, it can't be punted to the next labels Mar 25, 2023

weiznich mentioned this issue Apr 15, 2023

Prepare a 2.0.4 Release #3597

Closed

weiznich closed this as completed in ea5eda8 Apr 25, 2023

weiznich added a commit that referenced this issue Apr 25, 2023

Merge pull request #3607 from weiznich/fix/3566

15682ca

Fix #3566

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MultiConnection not usable for queries containing `Selectable` or boxed queries. #3566

MultiConnection not usable for queries containing `Selectable` or boxed queries. #3566

weiznich commented Mar 25, 2023 •

edited

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weiznich commented Mar 31, 2023

MultiConnection not usable for queries containing Selectable or boxed queries. #3566

MultiConnection not usable for queries containing Selectable or boxed queries. #3566

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weiznich commented Mar 25, 2023 • edited Loading

Setup

Versions

Feature Flags

Problem Description

What are you trying to accomplish?

What is the expected output?

What is the actual output?

Are you seeing any additional errors?

Checklist

weiznich commented Mar 31, 2023

MultiConnection not usable for queries containing `Selectable` or boxed queries. #3566

MultiConnection not usable for queries containing `Selectable` or boxed queries. #3566

weiznich commented Mar 25, 2023 •

edited

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