strange behaviour on saving rich join attributes

report.rb

class Report < ActiveRecord::Base

  has_many :report_parameters
  has_many :parameters, :through => :report_parameters do
    def push_with_attributes(monitored_parameter, rich_join_attributes)
      ReportParameter.send(:with_scope, :create => rich_join_attributes) { self << monitored_parameter }
    end
  end

  # "report_parameter_attributes"=>[{"value"=>"10", "parameter_id"=>"14"}]
  def report_parameter_attributes=(parameter_attributes)
    parameters.clear
    parameter_attributes.each do |attributes|
      parameters.push_with_attributes(Parameter.find(attributes[:parameter_id].to_i), { :value => attributes[:value] })
    end
  end

end

# if self(Report) is a new record join attribute is not stored upon creation.
# INSERT INTO `report_parameters` (`created_at`, `updated_at`, `report_id`, `value`, `parameter_id`) VALUES('2008-09-03 16:59:42', '2008-09-03 16:59:42', 5, NULL, 1)

# on update:
# INSERT INTO `report_parameters` (`created_at`, `updated_at`, `report_id`, `value`, `parameter_id`) VALUES('2008-09-03 17:00:06', '2008-09-03 17:00:06', 5, 'no', 1)