[Rails] Equivalent to observe_field in rails 3

Hi every one,
i need something like populating all the states
when a particular country selected. My exact requirement is, in my
project management tool, when a project is selected all of its
activities should be populated in the next drop down. I think before
rails 3 it can be done using observe_field tag. But how to do that in
rails 3 ( 3.0.10 to be precise) .

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