为了允许一个对象当且仅当一个特定的属性为nil时才有效，你可以使用“inclusion”而不是创建自己的方法。

validates :name, inclusion: { in: [nil] }

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这是针对Rails 3的。Rails 4解决方案要优雅得多：

validates :name, absence: true

型

class NoPresenceValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)                                   
    record.errors[attribute] << (options[:message] || 'must be blank') unless record.send(attribute).blank?
  end                                                                           
end    

validates :first_attribute, :presence => true, :if => "second_attribute.blank?"
validates :second_attribute, :presence => true, :if => "first_attribute.blank?"

validates :first_attribute, :no_presence => true, :if => "!second_attribute.blank?"
validates :second_attribute, :no_presence => true, :if => "!first_attribute.blank?"

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使用自定义验证。

validate :validate_method

# validate if which one required other should be blank
def validate_method
  errors.add(:field, :blank) if condition
end

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看起来：length => {：is => 0 }适合我的需要。

validates :first_attribute, :length => {:is => 0 }, :unless => "second_attribute.blank?"

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试试看：

validates :first_attribute, :presence => {:if => second_attribute.blank?}
validates :second_attribute, :presence => {:if => (first_attribute.blank? && second_attribute.blank? )}

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希望对你有帮助。