Ancestry allows the records of a ActiveRecord model to be organized in a tree
structure, using a single, intuitively formatted database column. It exposes
all the standard tree structure relations (ancestors, parent, root, children,
siblings, descendants) and all of them can be fetched in a single sql query.
Additional features are named_scopes, integrity checking, integrity restoration,
arrangement of (sub)tree into hashes and different strategies for dealing with