If I have a document with many embedded documents:

class Pack
  include Mongoid::Document

  embeds_many :items
  validates_presence_of :items
  accepts_nested_attributes_for :items,
                                allow_destroy: true
end

I can destroy all the items, with the standard rails _destroy option used with update_attributes:

@pack.update_attributes params[:pack]

At this point, the pack will have no items. If then I make a second edit to the pack, changing some other field, then the validator will activate and prevent the save.

Presumably this is because the items are individually destroyed, rather than perhaps being marked as to_destroy, and then actually destroyed with the parent document is saved.