Using polymorphic. With this, is possible to make a relation with any object.

diet.rb

# models/diet.rb
class Diet < ActiveRecord::Base
  include Todoable
end

prescription.rb

# models/prescription.rb
class Prescription < ActiveRecord::Base
  include Todoable
end

todo_item.rb

# models/todo_item.rb
class TodoItem < ActiveRecord::Base
  belongs_to :item, polymorphic: true

  def item_attributes
    self.item.attributes.keys - ["id", "created_at", "updated_at"]
  end
end

todoable.rb

# models/concerns/todoable.rb
module Todoable
  extend ActiveSupport::Concern

  included do
    has_one :todo_item, as: :item
    after_create :create_todo_item
  end

  def create_todo_item
    TodoItem.create(item_type: self.class.name, item_id: self.id)
  end
end