has_many :tags, :class_name => 'Tracetag', :foreign_key => 'gpx_id', :dependent => :delete_all
has_many :points, :class_name => 'Tracepoint', :foreign_key => 'gpx_id', :dependent => :delete_all
has_many :tags, :class_name => 'Tracetag', :foreign_key => 'gpx_id', :dependent => :delete_all
has_many :points, :class_name => 'Tracepoint', :foreign_key => 'gpx_id', :dependent => :delete_all