Selectable derive breaks embedded optional fields

[toxeus]

Setup

Versions

• Rust: 1.60.0
• Diesel: latest master
• Database: sqlite
• Operating System: Linux

Feature Flags

• diesel: sqlite

Problem Description

For a struct with an embedded optional struct the Selectable derive doesn't work correctly and, therefore, the compilation fails.
There is an unit test that tests exactly this scenario but the underlying diesel schema definition mistakenly is lacking the Nullable wrapper for the optional field.

What are you trying to accomplish?

Compile the code without errors.

What is the expected output?

Successful compilation.

What is the actual output?

Compiling diesel_derives v2.0.0-rc.0 (/home/user/src/diesel/diesel_derives)
error[E0277]: the trait bound expression::select_by::SelectBy<selectable::embedded_option::A, Sqlite>: load_dsl::private::CompatibleType<_, Sqlite> is not satisfied
--> diesel_derives/tests/selectable.rs:95:10
|
95 | .get_result(conn);
| ^^^^^^^^^^ the trait load_dsl::private::CompatibleType<_, Sqlite> is not implemented for expression::select_by::SelectBy<selectable::embedded_option::A, Sqlite>
|
= help: the following implementations were found:
<expression::select_by::SelectBy<U, DB> as load_dsl::private::CompatibleType<U, DB>>
= note: required because of the requirements on the impl of LoadQuery<'_, _, _> for SelectStatement<FromClause<my_structs_nullable::table>, diesel::query_builder::select_clause::SelectClause<expression::select_by::SelectBy<selectable::embedded_option::A, Sqlite>>>
note: required by a bound in get_result
--> /home/user/src/diesel/diesel/src/query_dsl/mod.rs:1652:15
|
1652 | Self: LoadQuery<'query, Conn, U>,
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in get_result

For more information about this error, try rustc --explain E0277.
error: could not compile diesel_derives due to previous error

Are you seeing any additional errors?

No.

Steps to reproduce

Add the following schema definition

table! {
    my_structs_nullable (foo) {
        foo -> Integer,
        bar -> Nullable<Integer>,
    }
}

and patch this test to use my_structs_nullable instead.

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

[weiznich]

Thanks for this bug report. I spend some time to investigate this.
I came to the conclusion that this is expected behaviour. Your inner field is not nullable in this case, while the corresponding field in your schema is nullable. This is a basic type mismatch, which always is considered as incompatible type. I would suggest to either wrap the inner type also in an Option or provide a manual Selectable impl
that does the "right" thing there.

See #3181 for more details.

Closed as not a bug.