如何正确设置这个嵌套关联?

I have the following models:

我有以下模型:

class MealPlan < ActiveRecord::Base
  has_many :food_contents
  has_many :foods,:through => :food_contents
  accepts_nested_attributes_for :food_contents
  attr_accessible :food_contents_attributes,:name
end

class Food < ActiveRecord::Base
  validates_presence_of :name,:protein,:carbs,:fats,:calories
  validates_numericality_of :protein,:carbs,:fats,:calories
end

class FoodContent < ActiveRecord::Base
  belongs_to :meal_plan
  belongs_to :food

  attr_accessible :food_id, :how_much,:meal_plan_id
  validates_presence_of :food,:meal_plan
end
class