instance method observer_class_for

# File activemodel/lib/active_model/observer_array.rb, line 74
      def observer_class_for(observer)
        return observer if observer.is_a?(Class)

        if observer.respond_to?(:to_sym) # string/symbol
          observer.to_s.camelize.constantize
        else
          raise ArgumentError, "#{observer} was not a class or a " +
            "lowercase, underscored class name as expected."
        end
      end