The hostgroup model defines ancestry twice, once directly and once via NestedAncestryCommon. This generates some log warnings (noticeable in production mode):

Creating scope :roots. Overwriting existing method Hostgroup.roots.
Creating scope :ancestors_of. Overwriting existing method Hostgroup.ancestors_of.
Creating scope :children_of. Overwriting existing method Hostgroup.children_of.
Creating scope :descendants_of. Overwriting existing method Hostgroup.descendants_of.
Creating scope :subtree_of. Overwriting existing method Hostgroup.subtree_of.
Creating scope :siblings_of. Overwriting existing method Hostgroup.siblings_of.
Creating scope :ordered_by_ancestry. Overwriting existing method Hostgroup.ordered_by_ancestry.
Creating scope :ordered_by_ancestry_and. Overwriting existing method Hostgroup.ordered_by_ancestry_and.
Creating scope :before_depth. Overwriting existing method Hostgroup.before_depth.
Creating scope :to_depth. Overwriting existing method Hostgroup.to_depth.
Creating scope :at_depth. Overwriting existing method Hostgroup.at_depth.
Creating scope :from_depth. Overwriting existing method Hostgroup.from_depth.
Creating scope :after_depth. Overwriting existing method Hostgroup.after_depth.

The has_ancestry definition should be removed from hostgroup.