gistfile1.txt

class Tagset
  include DataMapper::Resource
  include DataMapper::Timestamp
  include DataMapper::Serialize
  property :id,            Serial
  property :name,          String, :index => true, :nullable => false
  property :select_mode,   Boolean, :index => true, :default => true
  property :asserted_tags, Text, :default => '', :lazy => true
  property :created_at, DateTime
  property :updated_at, DateTime

  belongs_to :view, :nullable => true

  has n, :tags, :through => Resource

  ##
  # deassert the Tag with the given Tag name
  #
  # @param [String] name the name of a Tag object
  def deassert_tag(name)
    @@logger.debug{"deassert_tag(#{name})"}
    tag = self.tags.first(:name => name)
    unless tag.nil?
      joins = TagTagset.all(:tag_id => tag.id, :tagset_id => self.id)
      joins.each do |join|
        @@logger.debug{"destroying join TagTagset.all(:tag_id => #{tag.id}, :tagset_id => #{self.id})"}
        join.destroy
      end
      self.save
      self.tags.reload
      joins = TagTagset.all(:tag_id => tag.id, :tagset_id => self.id)
      unless joins.blank?
        @@logger.warn{"failed to deassert_tag(#{name}) joins => #{joins.inspect}"}
      end
    end
  end
end

When ran:

deassert_tag(OK)                                                                                                                      
destroying join TagTagset.all(:tag_id => 47, :tagset_id => 1)                                                                         
failed to deassert_tag(OK) joins => [#<TagTagset @tagset_id=1 @tag_id=47>]