class branch_good_issue_note(osv.osv):    
    _inherit = ['mail.thread', 'ir.needaction_mixin']
    _name = "branch.good.issue.note"

​_columns = {

'p_line':fields.one2many('branch.good.issue.note.line', 'line_id', 'Product Lines',
                      copy=True),

}

class branch_good_issue_note_line(osv.osv):  
    _name = 'branch.good.issue.note.line'
	​_columns = {
	​'product_viss':fields.function(_cal_viss_value, string='Viss', digits_compute=dp.get_precision('Cost Price'), type='float'),

}

​

def _cal_viss_value(self, cr, uid, ids, field_name, arg, context=None):
    res = {}
    uom_ratio = 1
    if context is None:
        context = {}
    for order in self.browse(cr, uid, ids, context=context):
        val1 = 0.0
        product = self.pool.get('product.product').browse(cr, uid, order.product_id.id, context=context)
        if product.uom_lines:
            if len(product.uom_lines.ids) == 3:
                cr.execute("select weight_ratio from product_uom where active = true and id=%s",(product.product_tmpl_id.report_uom_id.id,))
                weight_ratio = cr.fetchone()[0]
                if weight_ratio:
                    if order.issue_quantity > 0:
                        val1 = order.issue_quantity * (product.viss_value / weight_ratio)
                    else:
                        val1 = 0
            else:
                if product.product_tmpl_id.uom_id.id == order.product_uom.id:
                    cr.execute("select floor(round(1/factor,2)) as ratio from product_uom where active = true and id=%s", (product.product_tmpl_id.report_uom_id.id,))
                    bigger_qty = cr.fetchone()[0]
                    uom_ratio = bigger_qty
                if order.issue_quantity > 0:
                    val1 = order.issue_quantity * (product.viss_value / uom_ratio)
                else:
                    val1 = 0
        res[order.id] = val1
    return res

product_viss is not store=True

First, it is wrong, when I click "Edit" and click one line by one line. It's calcuation appear and When I click "Save" It is back to wrong value .Why???

Please Help Me !!!