Multiple table names in Queryable and QueryableByName

[snf]

In the issue #1314 it was discussed to add multiple table names for Insertable which got implemented in #3535

However, I'm having an issue with this because when I want to derive Insertable and Queryable/QueryableByName for a struct I need to use two different structs. I'm hacking around this implementing From one to the other to avoid too much code.

In the discussion I couldn't find any mention of why Queryable and QueryableByName were excluded.

• Queryable: it doesn't use the table name so the restriction could be lifted: https://github.com/diesel-rs/diesel/blob/master/diesel_derives/src/queryable.rs

• QueryableByName uses the table name to guess the types of the SQL columns but we could use the non-guessing path if there are multiple table names or compare that the type is the same in all the tables: https://github.com/diesel-rs/diesel/blob/master/diesel_derives/src/queryable_by_name.rs#L117 . Or other ideas?

There is also the discussion if we really want this as it may break backwards compatiblity once implemented if there are other features planned for the derives.

I can make a PR for either of those. I think implementing it for Queryable is trivial and it would just need a test.

[weiznich]

I think I would be fine with allowing that for #[derive(Queryable)] as table_name is not used there anyway.
For #[derive(QueryableByName)] we need to be a bit more careful as it might become something that's otherwise confusing for users, if it only "sometimes" works. I think we can workaround that by requiring that all fields have a #[diesel(sql_type = …)] attribute and otherwise generate an explicit error message explaining the problem.

[snf]

Added for Queryable in #3782

Should I leave this open for QueryableByName or close and re-open in case I get to work on that one?

[weiznich]

Its fine to just leave it open