Hello everyone , i created a module to inherit from the Sale module , the field i changed appers just fine on the interface here is my module :

ventes.py

from osv import fields,osv
import time
from datetime import datetime
from tools.translate import _


class ventes(osv.osv):


    _inherit='sale.order'
    _columns = {
      'prenom': fields.many2one('patient','Patient', required=True),


    }
ventes()

ventes_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
    <record model="ir.ui.view" id="view_sales_inherit">
        <field name="name">sale.order.form.inherit</field>
        <field name="model">sale.order</field>
        <field name="type">form</field>
        <field name="inherit_id" ref="sale.view_order_form">form</field>
        <field name="arch" type="xml">

            <field name="name" position="after">
                    <label for="prenom" class="oe_edit_only"/>
                       <h1>
                        <field name="prenom"/>
                       </h1>
            </field>

            <field name="partner_id" position="replace"/>


        </field>
    </record>   
 </data>
</openerp>

the problem is when i create a sale order , in the place of the client names i get the patients name ( witch is what i was looking for ) but the problem is when i want to add a product to that sale order i get the following error:

Error: Could not get field with name 'parent.partner_id' for onchange 'product_uom_change(parent.pricelist_id,product_id,product_uom_qty,product_uom,product_uos_qty,product_uos,name,parent.partner_id, False, False, parent.date_order, context)'

Plz help me, i dont know how to make it work with my patient's id and not the partner_id ??!!