_inherit = 'sale.order'

_columns = { 

'partner_ids': fields.many2many('res.partner', 'one_product_many_customer_rel','order_ids', 'partner_id', 'Customers'),

in xml file :

<xpath expr="//field[@name='partner_id']" position="after">

<!-- <field name='category_ids'/>-->

<field name="partner_ids" widget="many2many_tags" placeholder="e.g. Part Time"/>

</xpath>

""if you want to replace the partner _id in sale _order you need to change lots of base module codes regarding to that, it is a simply form to create that, and if you want to perform the task like make sales order, you need to customize it your own, it all are related to partner_id, so you should make your own code to perform the actions whatever you need to be perform.Dnt try to make the chances in base module its will produce some error.

prasanth.