Adding fields from a different object into a form in OpenERP

I would like to add fields from my product.product to my newly created Expense form (legacy_expense) which inherits hr.expense.expense. I created this module to edit the workflow of the Expense Management module - this I managed to do fine.

I'm new to OpenERP and am having trouble getting this to work. I tried inheriting the many2one product_id field in the form through my .py field and then get it to show in the xml but it always comes up with the error "XMLSyntaxError: attributes construct error, line 25, column 13"

I'm guessing my inheritance ins't correct and I have no idea how to relate the two objects. If someone could help me out with this it'll be great!

This is my customised legacy_expense.py file:

from openerp.osv import fields, osv

class legacy_expense(osv.osv):


        _inherit = 'hr.expense.expense'

        _columns = {
        'state': fields.selection([

        ('draft', 'New'),
        ('cancelled', 'Refused'),
        ('confirm', 'Waiting Approval'),
        ('done', 'Paid'),

        ], 'Order State', readonly= False, select=True),


        'product_id': fields.many2one('product.product','Product',required=True),


  }

legacy_expense()

This is the legacy_expense.xml file:

<?xml version="1.0" encoding="utf-8"?>
<openerp>
  <data>
    <record id = "view_expenses_form_custom" model="ir.ui.view">
        <field name="name">view.expenses.form.custom</field>
        <field name="model">hr.expense.expense</field>
        <field name="type">form</field>
        <field name="inherit_id" ref="hr_expense.view_expenses_form" />
        <field name="arch" type="xml">

                <data>
                    <header>
                        <button name="signal_draft_to_confirm" states="draft" string="Submit" type="workflow" groups="base.group_hr_user" />
                    </header>

                <header>
                        <button name="signal_confirm_to_done" states="confirm" string="Approve Expense" type="workflow" groups="base.group_hr_user" />
                    </header>

                <header>
                        <button name="signal_confirm_to_refused" states="confirm" string="Reject Expense" type="workflow" groups="base.group_hr_user" />
                    </header>
                </data>
        <xpath expr = "/form/sheet/group/group[2]/field[@name='user_valid']"position="after">
                <field name="product_id"/>
        </xpath>
        </field>
    </record>
</data>
</openerp>

Again, thanks in advance to anyone who can help!