Include Inserted Column Tuple type in Insertable trait

[LucaCappelletti94]

Backend

PostgreSQL, MySQL, SQLite, Others

What do you want to add to diesel?

When deriving Insertable, Diesel validates that struct fields correspond to table columns, but that information is not exposed in a reusable form. Unlike tables, which provide:

<MyTable as Table>::AllColumns

there is no equivalent for:

<NewUser as Insertable<users::table>>::InsertedColumns

Making such a tuple available would facilitate the definition of richer traits depending on that information in third party crates, without the need to redetermine the columns set.

Implementation notes

We could add a new associated type such as:

type InsertedColumns;

generated by the Insertable derive macro.

Additional details

No response

Checklist

• I have already looked over the issue tracker and the discussion forum for similar possible closed issues.

[weiznich]

Thanks for writing this proposal. I would like to see what you actually want to do with this information.

Otherwise I would like to note that adding a new associated type to an existing trait would be a breaking change. We don't plan any major release soon, so this is currently not an option.

[LucaCappelletti94]

I am working on the following pattern for builders/getters:

pub trait TypedColumn {
    type Type;
}

pub trait TypedColumns {}

// TypedColumns is implemented of tuples of typed columns via a macro

pub trait SetColumn<C: TypedColumn> {
    fn set(&mut self, value: C::Type);
}

pub trait GetColumn<C: TypedColumn> {
    fn get(&self) -> C::Type;
}

pub trait SetColumns<S: TypedColumns> {}
pub trait GetColumns<S: TypedColumns> {}

// SetColumns is implemented via a macro for all objects which
// implement `SetColumn` for all columns in the tuple `S`
// Analogously, `GetColumns` for all who implement `GetColumn`.

Regarding the breaking changeness of the proposal, we could easily rewrite it into an additional trait:

pub trait InsertableColumns: Insertable {
    type Columns;
}

And with this associated type, given an object I: InsertableColumns I can then require for a builder to have: SetColumns<I::Columns>, or a model of that table to have GetColumns<I::Columns>.

I am still working out a few aspects of this, but it really simplifies several constraints and I am now able to remove a significant amount of code generation I had written earlier on.

[weiznich]

Thanks for providing these details. I need a bit time thinking about this and also #4875, so expect some days until you get a proper response here.

[LucaCappelletti94]

No rush, at this time I am able to obtain the equivalent result with custom traits and derives, but I believe it should fit nicely in diesel.