struct Parent {
pub mut:
    id    int
    name  string
    child Child
    other Other
}

struct Child {
pub mut:
    id  int
    age int
}

struct Other {
pub mut:
    id   int
    name string
}

interface IdInterface {
mut:
    id int
}

fn insert_ids[T](val T) !T {
    mut clone := val

    $for field in T.fields {
        $if field.typ is $struct {
            clone.$(field.name) = insert_ids(val.$(field.name))!
        }
    }
    $if T is IdInterface {
        clone.id = 1
    } $else {
        return error('${T.name} does not have an id field!')
    }
    return clone
}

fn test_main() {
    inserted := insert_ids(Parent{
        name: 'test'
    })!
    assert inserted.child.id == 1
    assert inserted.other.id == 1
}