when I try to run below code i got this error 

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File "/home/user/odoo/addons/sale_validation/sale.py", line 708, in on_change_line_product

first_line_rec = self.browse(cr, uid, ids, context=context)[0]

File "/home/user/odoo/openerp/models.py", line 5476, in __getitem__

return self._browse(self.env, (self._ids[key],))

IndexError: tuple index out of range

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class purchase_order_line(osv.osv):

_inherit = 'purchase.order.line'

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

if context is None: context = {}

res = {}

line_num = 1

first_line_rec = self.browse(cr, uid, ids, context=context)[0]

for line_rec in first_line_rec.order_id.order_line:

res[line_rec.id] = line_num

line_num += 1

return res

_columns={

'line_no': fields.integer(string='Line Number'),

}

xml tag---------------

<field name="line_no" on_change="on_change_line_product(product_qty,context)"/>