Undefined method for `before_filter'

Try using

before_action :zero_authors_or_authenticated, only: [:new, :create]

Instead of

before_filter :zero_authors_or_authenticated, only: [:new, :create]

before_filter has been deprecated in Rails 5.0 and removed in 5.1.

If you are getting this error in Rails version < 5, check whether you have inherited your controller from action controller

class AuthorsController < ApplicationController
end

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