I want to add a field called loadnumber to a the purchase order form in the order lines but when i load the module i get the following error

Client Traceback (most recent call last):

  File "/home/kris/openerp/openerp/addons/web/http.py", line 195, in dispatch
    response["result"] = method(self, **self.params)
  File "/home/kris/openerp/openerp/addons/web/controllers/main.py", line 818, in authenticate
    req.session.authenticate(db, login, password, env)
  File "/home/kris/openerp/openerp/addons/web/session.py", line 116, in authenticate
    uid = self.proxy('common').authenticate(db, login, password, env)
  File "/home/kris/openerp/openerp/addons/web/session.py", line 31, in proxy_method
    result = self.session.send(self.service_name, method, *args)
  File "/home/kris/openerp/openerp/addons/web/session.py", line 104, in send
    raise xmlrpclib.Fault(openerp.tools.ustr(e), formatted_info)


Server Traceback (most recent call last):
  File "/home/kris/openerp/openerp/addons/web/session.py", line 90, in send
    return openerp.netsvc.dispatch_rpc(service_name, method, args)
  File "/home/kris/openerp/openerp/netsvc.py", line 295, in dispatch_rpc
    result = ExportService.getService(service_name).dispatch(method, params)
  File "/home/kris/openerp/openerp/service/web_services.py", line 425, in dispatch
    return fn(*params)
  File "/home/kris/openerp/openerp/service/web_services.py", line 436, in exp_authenticate
    res_users = pooler.get_pool(db).get('res.users')
  File "/home/kris/openerp/openerp/pooler.py", line 49, in get_pool
    return get_db_and_pool(db_name, force_demo, status, update_module)[1]
  File "/home/kris/openerp/openerp/pooler.py", line 33, in get_db_and_pool
    registry = RegistryManager.get(db_name, force_demo, status, update_module)
  File "/home/kris/openerp/openerp/modules/registry.py", line 192, in get
    update_module)
  File "/home/kris/openerp/openerp/modules/registry.py", line 214, in new
    openerp.modules.load_modules(registry.db, force_demo, status, update_module)
  File "/home/kris/openerp/openerp/modules/loading.py", line 343, in load_modules
    processed = load_marked_modules(cr, graph, states_to_load, force, status, report, loaded_modules, update_module)
  File "/home/kris/openerp/openerp/modules/loading.py", line 258, in load_marked_modules
    loaded, processed = load_module_graph(cr, graph, progressdict, report=report, skip_modules=loaded_modules, perform_checks=perform_checks)
  File "/home/kris/openerp/openerp/modules/loading.py", line 190, in load_module_graph
    load_data(module_name, idref, mode)
  File "/home/kris/openerp/openerp/modules/loading.py", line 75, in <lambda>
    load_data = lambda *args: _load_data(cr, *args, kind='data')
  File "/home/kris/openerp/openerp/modules/loading.py", line 126, in _load_data
    tools.convert_xml_import(cr, module_name, fp, idref, mode, noupdate, report)
  File "/home/kris/openerp/openerp/tools/convert.py", line 954, in convert_xml_import
    obj.parse(doc.getroot())
  File "/home/kris/openerp/openerp/tools/convert.py", line 847, in parse
    self._tags[rec.tag](self.cr, rec, n)
  File "/home/kris/openerp/openerp/tools/convert.py", line 814, in _tag_record
    id = self.pool.get('ir.model.data')._update(cr, self.uid, rec_model, self.module, res, rec_id or False, not self.isnoupdate(data_node), noupdate=self.isnoupdate(data_node), mode=self.mode, context=rec_context )
  File "/home/kris/openerp/openerp/addons/base/ir/ir_model.py", line 956, in _update
    res_id = model_obj.create(cr, uid, values, context=context)
  File "/home/kris/openerp/openerp/addons/base/ir/ir_ui_view.py", line 102, in create
    return super(osv.osv, self).create(cr, uid, values, context)
  File "/home/kris/openerp/openerp/osv/orm.py", line 4473, in create
    self._validate(cr, user, [id_new], context)
  File "/home/kris/openerp/openerp/osv/orm.py", line 1546, in _validate
    raise except_orm('ValidateError', '\n'.join(error_msgs))
except_orm: ('ValidateError', u'Fout opgetreden bij het valideren van veld(en) arch: Invalid XML for View Architecture!')

This is my code i have written

in the purchase_order.xml

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data>

        <record id="purchase_order_view_loadnumber" model="ir.ui.view">
            <field name="name">purchase order loadnumber</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='product_uom']" position="after">
                    <field name="loadnumber"/>
                </xpath>
            </field>
        </record>

    </data>
</openerp>

Purchase_order.py

from openerp.osv import osv, fields

class inkoop_bestelling(osv.Model):

    _inherit ='purchase.order.line'

    _columns= {
        'loadnumber' : fields.char('Laadnummer'),
        }

I've" been looking for hours and can't find what's wrong with it