How to get the type for a SQL numeric operation, like Add or Sub ?

[dessalines]

I'd like to use a subtract operation be used as a select_expression.

This gets the number of unread comments in a post: post_aggregates::comments - post_actions::read_comments_amount

This unfortunately can't use auto_type, and I also can't strictly define the type, since the Sub numeric operation is in a private module : diesel::expression::ops::numeric::Sub

My output type should be something like:

diesel::expression::ops::numeric::Sub<
  post_aggregates::comments,
  post_actions::read_comments_amount
>

Is there any way to get the type from a numeric operation?

Thanks in advance.

[weiznich]

It seems like these type definitions are still missing. I would appreciate a PR that adds them to diesel::dsl.

For #[auto_type] support see #4272 (not released yet, although I might just include that one in the next patch release)

[weiznich]

I just want to leave a note here that the 2.2.7 release contains a backport of #4272.

[dessalines]

Just to clarify, that is only for binary operations, correct? I played around for a few minutes trying to get auto_type for subtracting two Int8 columns working with 2.2.7, and failed.

[weiznich]

That's hard to answer without knowing the relevant code. The original PR contained at least one example that successfully compiled.

[dessalines]

I should probably add a unit test to back end diesel, but just anything like this.

This auto-type currently failing with 2.2.7:

#[diesel::dsl::auto_type]
fn subtract_test() -> _ {
  // Use any columns you like here
  post_aggregates::comments - post_actions::read_comments_amount 
}

[dessalines]

Gotcha. It might be a while until I can get to this, so I'll create an issue for it.