fix.rb

module ActiveAdmin
  module Filters
    module FormtasticAddons
      def klass
        @object.class
      end

      def polymorphic_foreign_type?(method)
        false
      end
    end
  end
end

Hi, I'm getting the same error with rails 4 and active admin for mongoid. Can you please explain how I can use this fix. Thanks

I put this in the config/initializers directory and it worked (as suggested by @bgwilson87)

Funfun's solution worked for me. Inserted fix.rb into the config/initializers directory, and I stopped getting the error. Thanks!