Not sure which versions are precisely affected.

See the attached file. One cannot define bi-directional relations like that:

class Library
  include Mongoid::Document
  has_many :relation, class_name: "Book", inverse_of: :relation
end

class Book
  include Mongoid::Document
  belongs_to :relation, class_name: "Library", inverse_of: :relation
end

It results with inconsistent behaviour. On the assignment attempt, one side of the relation will not be saved, and sometimes stack overflow error may be raised.