Here is my code 

class purchase_order_line(orm.Model):
    _inherit = 'purchase.order.line'
    _columns = {'packaging': fields.selection(((1, 1), (2, 2)), string='Packaging')}

xml

        <record id="purchase_purchase_order_packaging_form" model="ir.ui.view">
            <field name="name">purchase.purchase.order.packaging.form</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='order_line']//field[@name='price_unit']" position="before">
                    <field name="packaging"/>
                </xpath>
            </field>                  
        </record>
        <record id="purchase_purchase_order_line_packaging_form" model="ir.ui.view">
            <field name="name">purchase.purchase.order.line.packaging.form</field>
            <field name="model">purchase.order.line</field>
            <field name="inherit_id" ref="purchase.purchase_order_line_form"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='price_unit']" position="before">
                    <field name="packaging"/>
                </xpath>
            </field>                  
        </record>

I edit a line and select packaging 1,and save,in order line it was 1 too

but after I save , the value missed