"Set if unset" methods for ActiveValues

[PacificBird]

Motivation

ActiveValues lack a "null coalescing" operation that is common for ORMs in other languages (as well as Options in Rust with unwrap_or). Sometimes you need to set defaults in a model, but only when the field is unset. Currently, that requires making an if block, which is very ugly and verbose.

Proposed Solutions

A pair of new methods on ActiveValue, set_unset, set_unset_with, and set_unset_default, which will work sort of like unwrap_or, unwrap_or_else, and unwrap_or_default respectively.

For an ActiveValue<T>:

fn set_unset(&mut self, val: T) {
    match self {
        ActiveValue::Unset => self.set(val),
        _ => ();
    };
}

fn set_unset_with(&mut self, f: fn() -> T) {
    match self {
        ActiveValue::Unset => self.set(f()),
        _ => ();
    };
}


fn set_unset_default(&mut self)
where 
    T: Default
{
    match self {
        ActiveValue::Unset => self.set(T::default()),
        _ => ();
    };
}