there is a standard get_rate function which uses the currency_id and date as parameters to return the correct rate - either reference this or copy the code

This is from account/account_move_line.py

cur_obj = self.pool.get('res.currency')

tmp_amount = cur_obj.compute(cr, uid, line.account_id.company_id.currency_id.id, context.get('currency_id',False), abs(line.debit-line.credit), context={'date': line.date})

the function is compute not get_rate