Macro impl_preloadable_leaf 

macro_rules! impl_preloadable_leaf {
    ($ty:ty) => { ... };
}

Implement Preloadable for a hand-written model as a leaf association target.

#[model] implements Preloadable automatically. Use this macro for manually-defined models (those not using #[model]) that need to appear as the target of a #[belongs_to] / #[has_one] / #[has_many] on another model. A leaf target loads no associations of its own (its Spec is NoPreload), so it can be preloaded and wrapped in Preloaded but not nested into.

The type must implement diesel::Queryable/Selectable for its table.

autumn_web::impl_preloadable_leaf!(User);