Rails - Oct 2014 - Hello, How to choose values based on the selected above?

There is a form in it to be a field for entering regions(<select>)
On the basis of them should show the cities of these regions in the field 
below(<select>)

I think it ajax
But I'm not experienced this
Can you recommend what to read for it or (halpers rails for this task)

P.S Thanks for wathing

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