i have created a custom field "designation" in 'stock.picking' , now i want to get the value of the description of the product (in purchase.order) in that field  automatically when user push in the button " Reception by item"  (in my custom field)

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stock_view.xml

<?xml version="1.0" encoding="utf-8"?>

<openerp>

<data>

<record id="bl_stock_picking_tree" model="ir.ui.view">

<field name="name">bl.stock.picking.tree</field>

<field name="model">stock.picking</field>

<field name="inherit_id" ref="stock.vpicktree"/>

<field name="arch" type="xml">

<xpath expr="//field[@name='partner_id']" position="after">

<field name="num_bl"/>

<field name="designation"/>

</xpath>

</field>

</record>

<record id="bl_stock_picking_view_form" model="ir.ui.view">

<field name="name">bl.stock.picking.form</field>

<field name="model">stock.picking</field>

<field name="inherit_id" ref="stock.view_picking_form"/>

<field name="arch" type="xml">

<xpath expr="//field[@name='partner_id']" position="after">

<field name="num_bl"/>

<field name="designation"/>

</xpath>

</field>

        </record>

    </data>

</openerp>

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from openerp.osv import fields, osv

from openerp import tools

import openerp.addons.decimal_precision as dp

from openerp.tools.translate import _

class StockPicking(osv.osv):

_inherit = 'stock.picking'

def _get_designation(self,cr,uid,values,context=None):

designation=self.pool.get('purchase.order').browse(cr,uid,values.get('order_id')).product_uom.id

print StockQuantTreeDesignation

print values

return super (StockPicking,self).create(cr,uid,values,context)

_columns = {

'num_bl': fields.char( string="Numéro de BL fournisseur",required=True,select=True),

'designation': fields.char( string="Désignation",required=True,select=True,store=True, compute='_get_designation'),

}

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i have write this code but nothing is happened any help please