I am trying to add an on_change attribute to 'purchase_price' field in a sales_order form. (It might require sales_margin module so purchase_price appears).

I add:

    <record model="ir.ui.view" id="sale_margin_percent_5">
        <field name="name">change.purchase_price</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="sale.view_order_form"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='order_line']/tree//field[@name='purchase_price']" position="attributes">
                <attribute name="on_change">update_sale_price(sale_margin_percent, purchase_price, discount)</attribute>
            </xpath>
        </field>
    </record> 

But I get the following error:

Error details:
Element '<xpath expr="//field[@name='order_line']/tree//field[@name='purchase_price']">' can not be located in parent view

Context error:
View `change.purchase_price`
[view_id: 621, xml_id: n/a, model: sale.order, parent_id: 577]" while parsing /usr/lib/python2.7/dist-packages/openerp/addons/custom/sale_margin_percent/sale_margin_percent.xml:27, near
<record model="ir.ui.view" id="sale_margin_percent_5">
        <field name="name">change.purchase_price</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="sale.view_order_form"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='order_line']/tree//field[@name='purchase_price']" position="attributes">
                <attribute name="on_change">update_sale_price(sale_margin_percent, purchase_price, discount)</attribute>
            </xpath>
        </field>
    </record> 

Why "purchase_price" is not found? (I have added a similar on_change attribute for "price_unit" which works fine)