Implementing a custom predicate?

[ecclarke42]

In a case where a certain table has more than one identifying column (e.g. a user with id and unique email), I wanted to try to implement a custom predicate to filter for either one of the two.

For example, I'd love to be able to do something like this:

use crate::schema::users;

// Queryable, etc.
pub struct User {
    pub id: i32, // In practice, this is a id newtype
    pub email: String, 
    ...
}

pub enum UserIdentifier<'a> {
    Id(i32),
    Email(&'a str),
}

fn read(conn: &db::Connection, id: UserIdentifier) -> QueryResult<Self> {
    users::table.filter(id).first::<User>(conn)
}

rather than having to differentiate between users::id.eq(id) and users::email.eq(email), where some FilterDsl implementation handles the translation of id to each of those more diesel-y predicates.

But I'm not sure what the types on that impl would look like. There doesn't appear to be a Predicate trait to implement. And while it might be possible to impl FilterDsl<UserIdentifier> for users::table, that wouldn't cover any join query sources...

If feels possible, but maybe this is just one of those type shenanigans that isn't?

[weiznich]

I would suggest to either have an method on UserIdentifier which converts that type into a Box<dyn BoxableExpression<…, SqlType = Bool>> or to directly implement AsExpression<Bool> for UserIdentifier (Note: You need to manually implement this. The derive will not work for this case). Both allow reusing all existing FilterDsl implementations + allow to mix this with other predicates.

[ecclarke42]

AsExpression sounds like what I was looking for! But I'm guessing I'll still need to box something in the match on UserIdentifier, since the Eq expressions generated by each of those inner predicates have a different concrete type?

Edit: based on looking that the expressions module a bit closer, I should have noticed this before asking.