The model accout.bank.statement contains balance_end and balance_end_real.   balance_end computed when this fuction

def button_dummy(self, cr, uid, ids, context=None):
        return self.write(cr, uid, ids, {}, context=context)              

is called or if we click o save. I want to assign balance_end value(After computing) to balance_end_real field. How can i achieve this.