Allow to set fields to Default when constructing struct from query

[weiznich]

This commit adds a #[diesel(default)] field attribute to
#[derive(Queryable)], that allows a user to set a field to a default
value instead of loading it from the database. This is usefull if the
same struct is used to serve some API that requires additional
information to be added.

[sgrif]

👎 from me on this idea. It's too unclear what this does, and I don't think there's an easy way to make it obvious. I don't think it's obvious whether this ignores a database column, populates it on null, or changes the expected SQL type.

[weiznich]

Do you see a problem with this feature in general, or with the naming?
In the second case we could do something similar to serde and use #[diesel(skip)].

[weiznich]

@sgrif To point to a practical use of this, see this example from my wundergraph crate.
If there is any other way to do something similar that does not involve manually implementing Queryable I'm fine with using it.

[sgrif]

With the feature in general. I think this affects more things in non-obvious ways than #[serde(skip)] does.

[weiznich]

So is there any other way to solve this without manually implementing Queryable on each struct needing this?