How to use #[auto_type] with as_select()

[marcaddeo]

I'm trying to use #[auto_type] with a function that returns a select statement, which uses as_select() but I can't seem to get it working. Am I doing something wrong here?

Example code:

#[derive(Debug, Default, Queryable, Identifiable, Selectable, Insertable, Associations)]
#[diesel(table_name = crate::schema::post)]
#[diesel(belongs_to(UserRecord, foreign_key = user_id))]
#[diesel(check_for_backend(diesel::sqlite::Sqlite))]
pub struct PostRecord {
    pub id: i32,
    pub user_id: i32,
    pub content: String,
}

#[diesel::dsl::auto_type]
fn post_select() -> _ {
    post::table.select(PostRecord::as_select())
}

Which gives me an ambiguous associated type error that I can't seem to resolve.

On the other hand, this does work:

#[diesel::dsl::auto_type]
fn post_select() -> _ {
    post::table.select((post::id, post::user_id, post::content))
}

But obviously using as_select() is a bit more ergonomic.

This seems like it should work to me, but maybe I'm missing something.

[weiznich]

Selectable::as_select is one of the functions that is not supported by #[auto_type] so it is kind of expected to not work there. With the current limitations of what a proc-macro can do and with the current design of that function it's not possible to support it there as it has an unbounded generic type somehow needs to be specified.

The [documentation already] (https://docs.diesel.rs/2.2.x/diesel/dsl/attr.auto_type.html#limitations ) states that #[auto_type] doesn't support all of diesels DSL, although this function is missing from the list in the documentation. We would welcome a PR to add it there.

The workaround (provide an explicit type for this subexpression) listed in the documentation should also work here.

[marcaddeo]

Thanks! I was able to get it working as such:

#[diesel::dsl::auto_type]
fn post_select() -> _ {
    let as_select: AsSelect<PostRecord, Sqlite> = PostRecord::as_select();
    post::table.select((as_select,))
}

[weiznich]

Great to hear that you got it to work. I would advice to use select(as_select) instead of the nested tuple as the later can have interesting effects on deserialisation.