MongoidIssue.rb

class Plan
  include Mongoid::Document
  embeds_many :monthly_plans
end

class MonthlyPlan
  include Mongoid::Document
  embeds_many :planning_options
  embedded_in :plan, :inverse_of => :monthly_plans
end

class PlanningOption
  include Mongoid::Document
  embedded_in :monthly_plan, :inverse_of => :planning_options
end


def update
    plan = Plan.find(params[:plan_id])
    monthly_plan = plan.monthly_plans.find(params[:monthly_plan_id].to_i)
    planning_option = monthly_plan.planning_options.find(params[:id].to_i)
    planning_option.update_attributes!(params[:planning_option])
    respond_to do |format|
      format.js { render :nothing => true}
    end
end