I'm trying to extend the sale.order model to have a relation with an email chain model I've created in my custom module. So I created this:

    class sales_order_email_field(osv.Model):
        _name = 'sale.order'
        _inherit = 'sale.order'
        _columns = {
            'email_chain': fields.many2one(
                'sale.order_email.collection',
                'Email Chain',
                required=False
            )
        }

    sales_order_email_field()

and in my XML:

    <record id="view_res_sale_line_email_chain_form" model="ir.ui.view">
          <field name="name">sale.order.form.inherit</field>
          <field name="model">sale.order</field>
          <field name="type">form</field>
          <field name="priority" eval="8"/>
          <field name="inherit_id" ref="sale.view_order_form"/>
          <field name="arch" type="xml">
              <data>
                  <xpath expr="//field[@name='product_uom_qty']"
                         position="after">
                      <field name="email_chain"/>
                  </xpath>
              </data>
          </field>
      </record>

Buy I get an error message:

    Error details: Field `email_chain` does not exist

What am I doing wrong?