how do I test this?

gistfile1.rb

class ShoppingList < ActiveRecord::Base
  belongs_to :user
  has_many :shopping_list_ingredients
  
  has_many :ingredients, through: :shopping_list_ingredients
  
  
  
  serialize :current_mqrs
  

  # updates the cart in the case that the meal queue has changed
  def update_cart
    
    self.current_mqrs = [] if current_mqrs == nil
    
    mqrs = self.user.meal_queue.meal_queue_recipes
    slis = self.shopping_list_ingredients
    mqr_ids = mqrs.map(&:id)
    
    new_mqr_ids = mqr_ids - current_mqrs
    deleted_mqrs = current_mqrs - mqr_ids

    
    # check if meal queue has changed
    if mqr_ids.to_set != current_mqrs.to_set
      # if it has changed
      # find deleted mqrs
      # subtract quantities from items of deleted mqrs
      self.shopping_list_ingredients.each do |sli|
        
        sli_mqrs = sli.get_mqrs
        
        logger.info "ingredient is:"
        logger.info sli_mqrs.to_s
        
        sli_mqr_ids = sli_mqrs.map do |mqr| 
          logger
          mqr.fetch("id")
        end
        # find deleted mqrs in sli mqrs
        del_mqrs = sli_mqr_ids - (current_mqrs - deleted_mqrs)

        # subtract quantity of each deleted mqr from ingredient
        del_mqrs.each do |del_mqr|
          sli.quantity -= get_quantity(sli_mqrs, del_mqr)  
          # if quantity is 0, remove item
          if sli.quantity > 0.0
            sli_mqrs = sli.meal_queue_recipes
            sli_mqrs.delete_if { |sli_mqr| sli_mqr["id"] == del_mqr }
            sli.meal_queue_recipes = sli_mqrs
            sli.save
          else
            sli.destroy
          end
        end
      end
    
      self.shopping_list_ingredients(true)
      
      # find new mqrs
      new_mqrs = []
      mqrs.each do |mqr|
        new_mqrs << mqr if new_mqr_ids.include?(mqr.id)
      end
      #   add new ingredients / quantities
      new_mqrs.each do |mqr|
        mqr.recipe.recipe_ingredients.each do |ri|
          i = ri.ingredient.id
          new_sli = ShoppingListIngredient.find_or_create_by_shopping_list_id_and_ingredient_id(self.id, i)
          if new_sli.quantity == nil
            # new sli
            new_sli.quantity = ri.quantity
            new_sli.meal_queue_recipes = [{"quantity" => ri.quantity, "id" => mqr.id}]
            new_sli.save
            self.shopping_list_ingredients << new_sli
          else
            # existing sli
            new_sli.quantity += ri.quantity
            sli_mqrs = new_sli.meal_queue_recipes
            sli_mqrs << {"quantity" => ri.quantity, "id" => mqr.id}
            new_sli.meal_queue_recipes = sli_mqrs
            new_sli.save
          end
        end
      end
    end
    
    cmqrs = self.current_mqrs
    cmqrs = cmqrs.concat(new_mqr_ids)
    cmqrs.delete_if { |cmqr| deleted_mqrs.include?(cmqr) }
    self.current_mqrs = cmqrs
    self.save
    
    self.shopping_list_ingredients(true)
  end
  
  private
  
  # returns the quantity of an shopping_list_ingredient from a meal queue recipe
  def get_quantity(sli_mqrs, mqr_id)
    sli_mqrs.each do |mqr|
      return mqr["quantity"] if mqr["id"] == mqr_id
    end
    return nil
  end

end