I am trying to add a field to sale.order.form.

Specifically, I am trying to add x_sale_margin field to sale.order.lines so there is information on the sale margin for every sales order line. I have managed to complete the task by modifying everything in the GUI but now I am trying to get it working via module.

 I have tried this:

sale_margin_lines.py

# -*- coding: utf-8 -*-
from openerp import models, fields, api
class SaleMarginLines(models.Model):
        _inherit = 'sale.order.line',
        x_sale_margin = fields.Float('Margen de Ventas (%)', (4, 2)) 

sale_margin_lines.xml

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

     <record model="ir.ui.view" id="sale_margin_lines_sale_order">
        <field name="name">sale.order.margin.lines.view.form</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="sale.view_order_form"/>
        <field name="arch" type="xml">
            <xpath expr="//group[@name='sale_total']" position="after">
                <group name="sale_margin_grp">
                    <field name="x_sale_margin" groups="base.group_user"/>
                </group>
            </xpath>
        </field>
    </record>

    <record model="ir.ui.view" id="sale_margin_lines_sale_order_line">
        <field name="name">sale.order.line.margin.lines.view.form</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']/form//field[@name='price_unit']" position="after">
                <field name="x_sale_margin" groups="base.group_user"/>
            </xpath>
        </field>
    </record>

    <record model="ir.ui.view" id="sale_margin_lines_sale_order_line_form">
        <field name="name">sale.order.line.tree.margin.view.form</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='price_unit']" position="after">
                <field name="x_sale_margin" groups="base.group_user"/>
            </xpath>
        </field>
    </record>
  </data>
</openerp> 

Thanks,

P.D.

I also include the __init__.py and __openerp__.py files in case it helps to reproduce or to other people learning how to write modules.

__init__.py

 from . import sale_margin_lines 

__openerp__.py

 {
        'name': 'Sales Margin in Sales Orders',
        'description': 'Provides sales margin information in Sales Orders, both line by line and in total order',
        'author': 'E.M.',
        'depends': ['sale','sale_margin'],
        'data': ['sale_margin_lines.xml'],
}