Try this

from openerp import models,fields,api

class account_invoice_ext(models.Model):
        _inherit = 'account.invoice'

product_id=fields.Many2one("product.product",related="invoice_line.product_id",string="Product")
    
 

Your xml file

 

        <record id="view_account_invoice_filter_inherit" model="ir.ui.view">
            <field name="name">account.invoice.inherit.select</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.view_account_invoice_filter"/>
            <field name="arch" type="xml">
                <field name="period_id" position="after">
                    <field name="product_id"/>
                </field>
             </field>
         </record>

Hope this helps !!.