When doing a join, it's possible do write a custom handler. The handler can only build a custom ON predicate.

Why would you do it? I don't know :)

In all cases I can think of you get your ON predicate cheaper with on_sql.

However it's still possible:

# #![allow(unused_variables)]

#fn main() {
use toql::prelude::{JoinHandler, SqlExpr, SqlArg, ParameterMap, SqlBuilderError};

pub(crate) struct MyJoinHandler;

impl JoinHandler for MyJoinHandler {
    fn build_on_predicate(&self, on_predicate: SqlExpr, aux_params: &ParameterMap,)
     ->Result<SqlExpr, SqlBuilderError> {
        // Modify on_predicate based on aux_params
        Ok(on_predicate)
    }
}

// Getter function
pub fn my_join_handler() -> impl JoinHandler {
    MyJoinHandler {}
}
#}

Now map the getter function with

#[toql(join(), handler="my_join_handler")]
address: Address

And any join on Address will now call the join handler.