Support generic structures in FromQueryResult

[leviska]

Motivation

It seems that there should be no problem to write something like

#[derive(FromQueryResult)]
struct DateValue<T: TryGetable> {
  date: NaiveDate,
  value: T,
}

And then use it something like this

DateValue::<i32>::find_by_statement(..);
DateValue::<String>::find_by_statement(..);

I've tried doing that, and it seems like macro does not support it right now

error[E0107]: missing generics for struct `DateValue`
 --> stats/src/charts/insert.rs:6:12
  |
6 | pub struct DateValue<T: TryGetable> {
  |            ^^^^^^^^^ expected 1 generic argument
  |
note: struct defined here, with 1 generic parameter: `T`
 --> stats/src/charts/insert.rs:6:12
  |
6 | pub struct DateValue<T: TryGetable> {
  |            ^^^^^^^^^ -
help: add missing generic argument
  |
6 | pub struct DateValue<T><T: TryGetable> {
  |            ~~~~~~~~~~~~

Proposed Solutions

Add support to the macro

Even better would be is to add T: TryGetable in the impl<T: TryGetable> ... block, like rust does with Clone, Debug and etc, so it would require the type to implement trait only if somebody actually uses fromqueryresult functions

[tyt2y3]

It's a very interesting use case

[leviska]

Our use case is that we calculating different statistical values, and they could have different types (int, float or decimal).
The result type has the same structure - date and value.
So currently we have three structs, and I've came to idea to make it generic

[billy1624]

Released in 0.11.3