Multiple foreign keys with #[belongs_to], conflicting implementations of trait

[Boscop]

I have a table posts, and every post has an author (user_id) and a possible parent post that it replies to:

#[derive(Queryable, Identifiable, Clone, Associations, Debug)]
#[belongs_to(User)]
#[belongs_to(Post, foreign_key="parent")]
pub struct Post {
	pub id: i64,
	pub user_id: i64,
	pub parent: Option<i64>,
	// ...
}

But I get:

error[E0119]: conflicting implementations of trait diesel::SelectableExpression<diesel::query_source::InnerJoinSource<models::schema::__diesel_infer_schema::infer_posts::posts::table, models::schema::__diesel_infer_schema::infer_posts::posts::table>, diesel::types::BigInt> for type models::schema::__diesel_infer_schema::infer_posts::posts::columns::id:

What's the correct way to declare multiple foreign keys for this kind of usecase?

[Eijebong]

I don't think you can join more than 2 tables for now, have a look at #89

[sgrif]

@Eijebong You can't join to more than two tables in a single query, but having multiple associations on a single model is definitely supported. This is a bug.

[sgrif]

Ah, I see what's going on now. The issue isn't that there are multiple associations, it's that this is a self-referential association.

[Boscop]

Yeah, posts can have a parent post that they reply to.
Will it be possible in the future to write Post::belonging_to(&post) for a self-referential association like this?

[sgrif]

If by "Will it be possible in the future" you mean "is this a thing that I think is reasonable to do and want to support eventually?", then the answer is yes. If you mean "is this a thing you are actively working on and have a concrete timeline for?" then the answer is no

[sgrif]

As I was writing that I realized this is literally just an if statement in codegen. I'm 99% certain the only problem with it is the invocations of select_column_workaround!. This should be trivial to support.

[Boscop]

Yay!