instance variables with roar to dry up representers

inst_roar.rb

# use case:
# user has_many sport_sessions
# I want to return sport_sessions for a user week for week
# depending on given weekday


# module for DCI mixin
module UserRoles
  module TrainingWeekRepresenter

    include Roar::Representer::JSON
    require "#{RAILS_ROOT}/app/models/sport_session/training_day_representer_role.rb"

    property  :full_name
    collection :sport_sessions_for_week, :class => SportSession, :extend => SportSession::TrainingDayRepresenter

    def setup_for_representation(datetime)
      @datetime = datetime
    end

    def sport_sessions_for_week
      self.sport_sessions.where(:start_time.gt => (@datetime.beginning_of_week...@datetime.end_of_week))
    end

  end
end

# inside controller
# representer can now be used for any
# datetime and will return sport sessions
# for the corresponding week
def week_representation
  user = User.find(params[:id])
  user.extend_with_roles [:training_week_representer]
  user.setup_for_representation(params[:datetime]).to_json
end

Something like that would be cool:

user.to_json do ..
  :sport_sessons_for_week => user.sport_sessions_for_week(params[:datetime])

This implies that you must provide parameters from outside but it still does not rely on internal state. Of course, your solution works great but it makes your code vulnerable if a user is not following your life-cycle protocol (create user, extend, setup internals, render).