def strategy=(classifier)
      is_interchangeable?(classifier)
      if @strategy && classifier.category_count == 0
        previous_strategy = @strategy
        @strategy = classifier
        convert_categories_with_docs(previous_strategy)
      else
        @strategy = classifier
      end
    end
def strategy=(classifier)
      is_interchangeable?(classifier)
      if @strategy && classifier.category_count == 0
        previous_strategy = @strategy
        @strategy = classifier
        convert_categories_with_docs(previous_strategy)
      else
        @strategy = classifier
      end
    end
nodoc
 Changes classifier strategy and train new strategy if needed