in a many2many field  i try to get the sum of a column but it still 0, her s my code ;

in my classe : hr.expense.expense:

.........

def _get_expense_from_linemm(self, cr, uid, ids, context=None):

        return [line.expense1_id.id for line in self.pool.get('hr.expense.line').browse(cr, uid, ids, context=context)]

def _amountmm(self, cr, uid, ids, field_name, arg, context=None):

        res= {}

        for expense in self.browse(cr, uid, ids, context=context):

            total = 0.0

            for line in expense.line_ids:

                total += line.unit_amount * line.unit_quantity

            res[expense.id] = total

        return res

........

_column{

.......

'line1_ids': fields.many2many('hr.expense.line','hr_expense_rap','expenses_id','expense_line_id', string='expense'),

'amountmm': fields.function(_amountmm, string='Total Amount', digits_compute=dp.get_precision('Account'), 

            store={

                'hr.expense.line': (_get_expense_from_linemm, ['unit_amount','unit_quantity'], 10)

            }),

..........

}

in my class hr.expense.line :

_column{

'expense1_id': fields.many2many('hr.expense.expense','hr_expense_rap','expense_line_id','expenses_id', string='expense'),

}

any idea