Porting a complex query to 2.0.0

[ensc]

While porting to 2.0.0 I stumble about a huge monster query (original). A simplified version

#[macro_use] extern crate diesel as _;

use diesel::*;

diesel::table! {
    upload (id) {
        collection -> Nullable<Int4>,
        file -> Int4,
        ftime -> Nullable<Timestamp>,
        ftype -> Int2,
        id -> Int4,
        package -> Nullable<Int4>,
        project_ref -> Int4,
        scmrev -> Nullable<Text>,
        session -> Nullable<Int4>,
        tags -> Nullable<Text>,
        utime -> Timestamp,
    }
}

diesel::table! {
    permissions (id) {
        can_push -> Bool,
        can_read -> Bool,
        id -> Int4,
        project -> Int4,
        user -> Int4,
    }
}

diesel::table! {
    package (id) {
        id -> Int4,
        pn -> Nullable<Text>,
    }
}

diesel::table! {
    package_version (id) {
        id -> Int4,
        package -> Int4,
        pe -> Nullable<Text>,
        pr -> Nullable<Text>,
        pv -> Nullable<Text>,
    }
}

diesel::table! {
    project_ref (id) {
        baseref -> Nullable<Text>,
        branch -> Nullable<Text>,
        corename -> Nullable<Text>,
        distro -> Nullable<Text>,
        id -> Int4,
        lsb -> Nullable<Text>,
        project -> Int4,
        user -> Int4,
    }
}

diesel::table! {
    file (id) {
        ctime -> Timestamp,
        data -> Int4,
        id -> Int4,
        rcount -> Int4,
        rtime -> Nullable<Timestamp>,
        wcount -> Int4,
        wtime -> Timestamp,
    }
}

diesel::table! {
    file_data (id) {
        compr_type -> Int2,
        digest -> Text,
        digest_type -> Int2,
        fsize -> Int8,
        id -> Int4,
        storage_name -> Text,
    }
}

diesel::joinable!(upload -> file (file));
diesel::joinable!(upload -> package_version (package));
diesel::joinable!(upload -> project_ref (project_ref));
diesel::joinable!(package_version -> package (package));
diesel::joinable!(file -> file_data (data));
diesel::joinable!(permissions -> project_ref (project));

diesel::allow_tables_to_appear_in_same_query!(
    file,
    file_data,
    package,
    package_version,
    permissions,
    project_ref,
    upload,
);

#[derive(Queryable, Debug)]
pub struct CandidateResult
{
    pub upload_id:	i32,
    pub upload_file:	i32,

    pub file_data_id:	i32,
    pub pn:		Option<String>,
    pub pnve_id:	Option<i32>,

    pub project_ref:	i32,

    pub project:	i32,
    pub uploader:	i32,
}

fn foo(conn: &diesel::pg::PgConnection)
{
    let _query = upload::table
	.inner_join(project_ref::table
		    .inner_join(permissions::table))
	.left_join(package_version::table
		   .inner_join(package::table))
	.inner_join(file::table
		    .inner_join(file_data::table))
	.select((upload::dsl::id, upload::dsl::file,
		 file::dsl::data,
		 package::dsl::pn,
		 upload::dsl::package,
		 project_ref::dsl::id,

		 project_ref::dsl::project,
		 project_ref::dsl::user))

	.load::<CandidateResult>(conn);
}

fn main() {
}

builds fine with 1.4.6 but fails on diesel master with

error[E0271]: type mismatch resolving `<SelectStatement<JoinOn<_::query_source::joins::Join<package_version::table, package::table, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<package_version::columns::package>, _::expression::nullable::Nullable<package::columns::id>>>>> as AppearsInFromClause<package::table>>::Count == _::query_source::Never`
   --> src/main.rs:126:3
    |
126 |     .select((upload::dsl::id, upload::dsl::file,
    |      ^^^^^^ expected struct `_::query_source::Once`, found struct `_::query_source::Never`
    |
    = note: required because of the requirements on the impl of `SelectableExpression<_::query_source::joins::Join<JoinOn<_::query_source::joins::Join<upload::table, SelectStatement<JoinOn<_::query_source::joins::Join<project_ref::table, permissions::table, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<permissions::columns::project>, _::expression::nullable::Nullable<project_ref::columns::id>>>>>, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<upload::columns::project_ref>, _::expression::nullable::Nullable<project_ref::columns::id>>>>, SelectStatement<JoinOn<_::query_source::joins::Join<package_version::table, package::table, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<package_version::columns::package>, _::expression::nullable::Nullable<package::columns::id>>>>>, LeftOuter>>` for `package::columns::pn`
    = note: 4 redundant requirements hidden
    = note: required because of the requirements on the impl of `SelectableExpression<JoinOn<_::query_source::joins::Join<JoinOn<_::query_source::joins::Join<JoinOn<_::query_source::joins::Join<upload::table, SelectStatement<JoinOn<_::query_source::joins::Join<project_ref::table, permissions::table, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<permissions::columns::project>, _::expression::nullable::Nullable<project_ref::columns::id>>>>>, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<upload::columns::project_ref>, _::expression::nullable::Nullable<project_ref::columns::id>>>>, SelectStatement<JoinOn<_::query_source::joins::Join<package_version::table, package::table, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<package_version::columns::package>, _::expression::nullable::Nullable<package::columns::id>>>>>, LeftOuter>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<upload::columns::package>, _::expression::nullable::Nullable<package_version::columns::id>>>>, SelectStatement<JoinOn<_::query_source::joins::Join<file::table, file_data::table, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<file::columns::data>, _::expression::nullable::Nullable<file_data::columns::id>>>>>, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<upload::columns::file>, _::expression::nullable::Nullable<file::columns::id>>>>>` for `(upload::columns::id, upload::columns::file, file::columns::data, package::columns::pn, upload::columns::package, project_ref::columns::id, project_ref::columns::project, project_ref::columns::user)`
    = note: required because of the requirements on the impl of `SelectDsl<(upload::columns::id, upload::columns::file, file::columns::data, package::columns::pn, upload::columns::package, project_ref::columns::id, project_ref::columns::project, project_ref::columns::user)>` for `SelectStatement<JoinOn<_::query_source::joins::Join<JoinOn<_::query_source::joins::Join<JoinOn<_::query_source::joins::Join<upload::table, SelectStatement<JoinOn<_::query_source::joins::Join<project_ref::table, permissions::table, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<permissions::columns::project>, _::expression::nullable::Nullable<project_ref::columns::id>>>>>, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<upload::columns::project_ref>, _::expression::nullable::Nullable<project_ref::columns::id>>>>, SelectStatement<JoinOn<_::query_source::joins::Join<package_version::table, package::table, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<package_version::columns::package>, _::expression::nullable::Nullable<package::columns::id>>>>>, LeftOuter>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<upload::columns::package>, _::expression::nullable::Nullable<package_version::columns::id>>>>, SelectStatement<JoinOn<_::query_source::joins::Join<file::table, file_data::table, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<file::columns::data>, _::expression::nullable::Nullable<file_data::columns::id>>>>>, Inner>, Grouped<_::expression::operators::Eq<_::expression::nullable::Nullable<upload::columns::file>, _::expression::nullable::Nullable<file::columns::id>>>>>`

How can I fix this?

[weiznich]

As a blind guess I would say you need to change package​::​dsl​::​pn to package​::​dsl​::​pn.nullable() as this column coming in via a left join. This implies that you need to change the corresponding field to an option. This was accepted by diesel 1.x, because of a bug.