update_attributes.rb

class Routine
  include Mongoid::Document
  field :name, :type => String
  field :description, :type => String
  
  embeds_many :workouts

  accepts_nested_attributes_for :workouts, :allow_destroy => true
end

class Workout
  include Mongoid::Document
  field :title, :type => String
  field :notes, :type => String
  
  references_many :workout_sets
  embedded_in :routine, :inverse_of => :workouts
  
  accepts_nested_attributes_for :workout_sets, :allow_destroy => true
end

class WorkoutSet
  include Mongoid::Document
  field :reps, :type => Integer
  field :sets, :type => Integer
  
  referenced_in :workout
  referenced_in :exercise
end


{"routine"=>{"name"=>"another workout", "description"=>"good workout", "workouts_attributes"=>{"0"=>{"title"=>"day1", "notes"=>"asdf", "_destroy"=>"false", "workout_sets_attributes"=>{"0"=>{"_destroy"=>"false", "exercise_id"=>"4d2698a83c850c399a000001", "sets"=>"2", "reps"=>"3"}}}}}, "utf8"=>"✓", "authenticity_token"=>"tpptQ9A7nnfU8lr65RhDtMBQ4HmEHtZm0g6i3XAKfMQ=", "id"=>"4d46db123c850c378f000001"}