class Band
  include Mongoid::Document
  has_and_belongs_to_many :tags, inverse_of: nil
end

class Tag
  include Mongoid::Document
  field :name, type: String
end

If I has something like that and I delete a Tag document, the document ID persists in the Band array of tags, if I do a Band.first.tags it shows only the tags that still exists but if I try Band.first.tag_ids it shows all IDS they currently exists or not