class ActiveModel::Errors

person.errors.include?(:age) # => false
person.errors.include?(:name) # => true
person.errors.messages # => {:name=>["cannot be nil"]}

+attribute+, +false+ otherwise.
Returns +true+ if the error messages include an error for the given key

def include?(attribute)
  @errors.any? { |error|
    error.match?(attribute.to_sym)
  }
end