why select helper does not auto show the selected value?

This code works:

select("questionnaire", "is_active", options_for_select([["no", false], ["yes", true]], @questionnaire.is_active) )

But this will not:

select("questionnaire", "is_active", options_for_select([["no", false], ["yes", true]]) )

Why doesn't rails know to handle the selected value by itself? What's the logic?