How would I unit test add_to_shopping_list?

gistfile1.rb

class MealQueueRecipe < ActiveRecord::Base
  belongs_to :meal_queue
  belongs_to :recipe
  
  after_create :add_to_shopping_list
  after_destroy :remove_from_shopping_list
  
  def add_to_shopping_list
    sl = get_shopping_list
    sl.add_mqr(self)
  end
  
  def remove_from_shopping_list
    sl = get_shopping_list
    sl.remove_mqr(self)
  end
  
  
  private
  
  def get_shopping_list
    self.meal_queue.user.shopping_list
  end
end