How do I validate that a question has at least one answer?

My question model h开发者_Python百科as_many :answers in Rails 3. I want to be able to validate that each question has at least one answer.

I tried the following code:

validate :must_have_answer

def must_have_answer
  if answers.empty? or answers.all?{ |example| example.marked_for_destruction? }
    errors.add_to_base{ "Must provide at least one answer"}       
  end
end

I got the following error:

wrong number of arguments (0 for 1)

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Try passing a string instead of a block to errors.add_to_base

errors.add_to_base "Must provide at least one answer"