class Product < ActiveRecord::Base
  #... all your other stuff
  validate :require_two_options

  private
    def require_two_options
      errors.add(:base, "You must provide at least two options") if options.size < 2
    end
end

只是一个关于karmajunkie回答考虑：我会使用size而不是count，因为如果一些构建的（未保存的）嵌套对象有错误，它将不会被考虑（它还没有在数据库中）。

class Product < ActiveRecord::Base
  #... all your other stuff
  validate :require_two_options

  private
    def require_two_options
      errors.add(:base, "You must provide at least two options") if options.size < 2
    end
end

如果您的表单允许删除记录，则.size将无法工作，因为它包含标记为销毁的记录。
我的解决方案是：

validate :require_two_options

private
 def require_two_options
    i = 0
    product_options.each do |option|
      i += 1 unless option.marked_for_destruction?
    end
    errors.add(:base, "You must provide at least two option") if i < 2
 end

更整洁的代码，使用Rails 5测试：

class Product < ActiveRecord::Base
  OPTIONS_SIZE_MIN = 2
  validate :require_two_options

  private

  def options_count_valid?
    options.reject(&:marked_for_destruction?).size >= OPTIONS_SIZE_MIN
  end

  def require_two_options
    errors.add(:base, 'You must provide at least two options') unless options_count_valid?
  end
end

我想知道为什么没有人提到这个简单的解决方案。只需将此添加到您的父类：

class Product < ActiveRecord::Base

  ...
  
  validates :options, :length => { :minimum => 2 }

end

在Rails 5.2.8（可能还有更高版本）中工作对我来说很有魅力。