AsChangeSet: adding a method which can easily check if there is no change to save

[dajoha]

When updating a table with diesel::update(), you can easily get this error:

There are no changes to save. This query cannot be built

Ok, so for a little struct like this:

#[derive(AsChangeSet)]
struct Person {
    name: Option<String>,
    age: Option<i8>,
}

, I just need to add a preliminary guard test, something like this:

if matches!(person, Person { name: None, age: None }) {
    return;
}

But for a bigger struct it can become tedious.

What about adding a method like has_changes() to the trait AsChangeSet, which would be automatically implemented when auto-deriving? It would return false when all the struct fields are None; or to be more precise, it would return false each time the error "There are no changes to save. This query cannot be built" would be triggered.

I am really not aware of the ins and outs of such a question, but it would be really nice, to my user's point of view:)

What do think about it?

A related issue: #1996

[weiznich]

Thanks for filling this feature request.
I'm happy to read a concrete proposal for this. I would expect that such an proposal explains how exactly AsChangeset should be changed, how that will be supported in the derive and how that will be supported in a backward compatible way.

As an intermediate solution: You can always match on the error returned by diesel to detect this case + ignore that error.

[dajoha]

Thank you for your reply; I posted this suggestion a bit too fast I must admit:)

This would be a nonsense to add such a method in the trait AsChangeset.

I guess it would not be very difficult to implement it in the derive macro, but it would force every people who implemented the trait manually to add this method, with no reason.

It's just totally useless, even more because it can simply be implemented by an external crate, like https://crates.io/crates/is_empty seems to do (not tested).