Showing validation/error reporting problem with associated Mongoid models

mongoid-models-summary.rb

class Artist
  include Mongoid::Document
  has_many :history_events, autosave: true, validate: true
end

class HistoryEvent
  include Mongoid::Document
  belongs_to :artist

  validates_presence_of :headline
end
# when updating artist and a new history event has a blank headline, the error is reported properly
# when updating artist and an existing history events' headline is removed, the document does not save, but the error is not reported.