my filelds are

 def _get_total(self, cr, uid, ids, name, unknow_none, context=None):
        res = {}
        for record in self.browse(cr, uid, ids, context=context):
            total = 0.0
            for line in record.cost_ids:
                total += line.amount
            res[record.id] = total
        return res  
_columns = {
 'total': fields.function(_get_total, string='Total', type='float', method=True, store=True),
 'cost_amount': fields.related('total','cost_id', 'amount', string='Amount',type='float',store=True),  
}

it got an error like TypeError: 'float' object has no attribute '__getitem__'

pls anybody can help me regards jamshid k