Ruby on Rails, 2 dimensional form

So I have a form that has sales budgets by user, displayed in a grid e.g

User  Jan   Feb   Mar
Joe   100   200   300
Bob   300   240   140

The budget value field needs to identify the user and the month it is for, so on the server side I can allocate the values correctly, something like user[12]_month[1].

How do I build these form names, and then extract the identifying data server side?