How to use complex expression in conflict update?

[Cofecit]

Hello everyone, I have a MySQL upsert SQL like:

INSERT INTO `test`.`t_json_test` (
    `id`,
    `age`,
    `username`
)
VALUES (?, ?)
ON DUPLICATE KEY UPDATE
`age` = VALUES(`age`),
`username`  = IF(VALUES(`username`) IS NOT NULL,
              JSON_MERGE_PATCH(
                      IF(`t_json_test`.`username` IS NOT NULL,
                            `t_json_test`.`username`,
                            JSON_OBJECT()),
                      VALUES(`username`)),
              `t_json_test`.`username`)

how to translate it into Sea-Query update conflict?

I have tried like this:

let mut data = Map::new();
data.insert("abcd".into(), serde_json::Value::String("1111".to_string()));
let m = model::t_json_test::ActiveModel {
    id: ActiveValue::Set(2),
    age: ActiveValue::Set(Some(22)),
    username: ActiveValue::Set(Some(Json::Object(data))),
};

let insert = t_json_test::Entity::insert(m)
    .on_conflict(
        sea_query::OnConflict::column(t_json_test::Column::Id)
            .update_column(t_json_test::Column::Age)
            .value(t_json_test::Column::Username,
                   Expr::cust_with_exprs(
                       "IF(? IS NOT NULL, ?, ?)",
                       [
                           Expr::cust_with_expr("VALUES(?)", Expr::col(t_json_test::Column::Username)).into(),
                           Expr::cust_with_exprs(
                               "JSON_MERGE_PATCH(IF(? IS NOT NULL, ?, JSON_OBJECT()), ?)",
                               [
                                   Expr::col((t_json_test::Table::Name, t_json_test::Column::Username)).into(),
                                   Expr::col((t_json_test::Table::Name, t_json_test::Column::Username)).into(),
                                   Expr::cust_with_expr("VALUES(?)", Expr::col(t_json_test::Column::Username)).into(),
                               ],
                           ),
                           Expr::col((t_json_test::Table::Name, t_json_test::Column::Username)).into()
                       ]))
            .to_owned()
    );

// Output SQL: "INSERT INTO `t_json_test` (`id`, `age`, `username`) VALUES (2, 22, '{\\\"abcd\\\":\\\"1111\\\"}') ON DUPLICATE KEY UPDATE `age` = VALUES(`age`), `username` = IF(VALUES(`username`) IS NOT NULL, JSON_MERGE_PATCH(IF(`t_json_test`.`username` IS NOT NULL, `t_json_test`.`username`, JSON_OBJECT()), VALUES(`username`)), `t_json_test`.`username`)"
println!("SQL: {:?}", insert.build(DatabaseBackend::MySql).to_string());

a slight deficiency is no VALUES(col) in Expr, and MySQL note me that VALUES function is deprecated and will be removed in a future release. Please use an alias (INSERT INTO ... VALUES (...) AS alias) and replace VALUES(col) in the ON DUPLICATE KEY UPDATE clause with alias.col instead.