Function child_by_id 

pub fn child_by_id<R>(
    state: &<R as Resource>::State,
    doc: &AutoCommit,
    id: &str,
) -> Option<Resource>
where
    R: Resource,

The nested resource id of state, read from doc with id demanded when state has not loaded it: an item past what a view shows or a tree pages still resolves by id (Resource::child, Resource::load).